Are you talking a MH or truck? The answers will differ.

We have owned a gas motorhome on the Workhorse W-22 chassis using a 340 HP Vortec (GM) engine and a 5 speed Allison transmission, which is a front engine motorhome.  We now own a diesel motorhome on a Spartan Mountain Master chassis using a Cummins 370HP ISL (8.9 liter) engine and 6 speed Allison transmission, which is a rear engine motorhome (diesel pusher).  Our diesel has air brakes and suspension, the gas was disc brakes and a conventional suspension.  The diesel has an engine brake, our gas did not.   That lays out our direct experience.
You can buy diesel pullers (FRED from Freightliner is the newest version) and you can buy gas pushers (Workhorse UFO is the newest version).  You can buy diesels with no air brakes or air suspension, like the FRED.
Diesel engines will get better mileage if the motorhomes are about the same weight, but most of the diesels are much heavier motorhomes so it is not always true.  Diesel engines will typically last longer, 500,000 or more miles while the current gas motorhomes will last 150,000 or more miles.
Diesel engines are a price premium, they will cost more, often much more.
At the same horsepower a diesel will have more torque, so it will pull a load better.  At the same horsepower a gas will normally out accelerate a diesel, it is just the way the engines behave.  You rarely find the same horsepower in a similar gas or diesel motorhome, the diesel will normally be lower horsepower per the total weight of the motorhome.   Our 40ft 33,000lb diesel gets about the same mileage (7-7.5mpg) as our 35ft 22,000lb gas got.
Air ride is better, no doubt, but not all diesels have it.   We really like our air ride.
Engine braking or transmission braking is way better, most diesel pushers come with it and some gas motorhomes now have transmission braking.   We really like having engine braking and wish we would have had the option with our gas motorhome.
In the February 2008 Motorhome magazine they compared identically equipped motorhomes using a Freightliner diesel pusher chassis, a Winnebago Destination, and a gas pusher using a Workhorse UFO chassis, an Itasca Latitude.   The diesel was a 300 HP Cummins ISB (6.7 liter) and the Workhorse used the 340HP Vortec.  The diesel averaged 10.92 mpg and the gas averaged 7.57.  The gas was a bit faster and caught the diesel going up a 7% grade. 

In my opinion, the larger the MH the more you need to consider diesel. We put many miles on a smaller MH with a gas engine and had no problems, but I sure wouldn't want a gas engine in our present 40 footer.

Jim

Hi Jim,
  what do you consider a smaleer MH

Jim: How old was your 35' gas motorhome?  Was it a Ford?

Anyway our Vortec gas engine had no trouble pulling us over the Rockies and Cascades and various other passes with a 35' motorhome.  We passed many older diesels going up those grades.  Going down we had to be more careful because that 2002 gas motorhome did not have any kind of engine or transmission braking, but we were careful and made it.  The most memorable downgrade is Cloundcroft, NM to Alamogordo, which was 16 miles of 6% grade losing 4,200 feet of elevation.  The newest Workhorse gas chassis come with transmission braking.
We do like our diesel, but I won't discount gas based on older gas motorhomes.  I know many who have owned older diesels and 190 hp is too little.

bjoyce,

It was a 1998 Winnebago Chieftain with the 454 Chev engine on the P 30 Chassis.

I just felt that on the steeper grades, it really had to struggle at times. I especially didn't like the engine screaming at the high RPMs that it was required to run at to develop the power. I hardly ever pushed the engine to it's limits, but even without giving it full throttle, there was a lot of noise coming from under that doghouse. Like I said, I know some of the newer engines have more power, but I really do think that anything over about 35' is better off with a diesel.

I really didn't have too much trouble with the downgrades. I would just start out real slow at the top in a low gear, and slowly head down. Really didn't have to use much brakes. Just tapped them at times if the engine RPM got too high.

The MH we have now is an older 40' Beaver Marquis with a 325 HP CAT engine with a 2 stage Jake-brake and is quite good in the mountains. It's also way more comfortable to drive with the air ride, and being a heavier coach, the wind doesn't bother it as much.

Of course we're not the fastest thing out there, but we're not in a hurry either.

-- Edited by Jim01 at 12:53, 2008-02-23

There is a lot of variation out there, especially with rear engine gas and front engine diesels now available.  Our front engine gas motorhome had a good ride and didn't cause us any motion sickness, plus we could still talk over the engine sound.  Only diesel pushers with air ride are smoother riding and quieter, not all diesels have air ride and not all are pushers.
It is was ride quality I would have an Alpine Coach, who makes their own diesel pusher chassis, it test drove wonderfully and it used mechanical brakes and not air brakes.  Too bad we did not like the basement layout, the floorplan choices, and the more than $40K higher cost than the Dutch Star diesel pusher we bought.

Some have mentioned that each type of engine has advantages and disadvantages.  Some have also suggested that suspension etc may affect your experience.  Our experience is not with motorhomes but our research on trucks indicates transmission, suspension, hitches, and weight distribution are big factors in the overall experience.  It seems that there are various combinations to achieve different results and they perform differently.  Fuel efficient high torque, low horsepower engines do the job but don't satisfy everyone.  If how fast you go up the mountain or down the mountain is more important then a different combination works for you.  If accelerating on and off the freeway or interstate is more important then you need to get the right combination.  Transmission brakes, exhaust brakes, tow haul, disk brakes, automatic, standard, all have significant effects on performance but the most important thing is your expectations and habits.  Buy what you need to get what you want.

Strange but you can make those choices and you will be happier with the result.  The combinations of packages provide a wide variety of results and do make a difference.  We have friends who recently traded motor homes to get a better engine transmission combination from their perspective.  The motorhome was almost the same otherwise.  It is worth thinking about before you buy.
 
We are very happy with an 8.1L gas engine with an allison transmission.  We could also be happy with a diesel with the right package.

Larry and Jacki

Hello; Now that I have learned a lot about MH diesel or gas. Great info! Would someone help me decide about a truck...Diesel or Gas? I will have a truck camper, only 3000lbs and tow a trailer about 4000 lbs.

thanks