Ok I just thought I would post this topic to provide info to anyone interested about my addition of a propane fired instant water heater.  I did a lot of research on here about the subject and only really found some cursory information on the topic.  I pm'd Hit The Road Jack (rv.net) for some info and he was very helpful.  So here is what I did.  I purchased an Excel tankless instant water heater model number 10002P1 and paid $327 for it including shipping. It heats 1.6 GPM with a 45 degree rise in temp and is a ventless model that does not require a flue.  It uses 2 D batteries for the spark ignition and only requires three connections water in, hot water out and propane in.  I installed it in the basement of my Sierra Sport 38SP toy hauler. It requires a standard propane regulator (like the one already on your RV) that provides 11" WC and 1/2 PSI.  It will heat 950 gallons of water on one bbq size propane tank. I will add some pics but have not had a chance to take some yet.  I tapped into the cold water input line to my existing suburban 6 gal DSI water heater and then ran a PEX water line to the input of the new excel heater.  I then ran another PEX line from the output of the excel unit to the input of the original water heater. I also installed a bypass valve at that end so I could bypass one or both of my water heaters in case there was a problem or for winterization.  So in summary the water comes into the tankless water heater and is heated 45 degrees. It then flows to the regular 6 gal tanked water heater and then out to my hot water faucets. If my input water temp is above 60 degrees then the tank unit does not even fire up because the tankless is providing hot enough water. The tankless is rated to 9200 Ft elevation and I am currently using it at 8500 feet with no issues.  My basement is a little small so I purchased a fridge fan kit and cut a hole in the side of my rig right above the basement door and added a dryer vent cover and the fan right behind it.  This is the kit I bought Valtera Fidge Cool. It has two temp switches so I mounted the low temp one to the metal cover of the tankless unit and now anytime the tankless unit comes on within about 20 seconds the vent fan comes on and pushes open the louvers on the dryer vent. I drilled a 1.5 inch hole in the basement floor directly below the gas line connection for the instant water heater and ran a flexible gas line out the bottom to a separate bbq size tank and regulator.  Later I am planning on making a quick connect to my existing propane line but so far I kind of like having the sep tank. As soon as you turn on the hot water the heater lights up and as soon as you turn off the hot water the unit shuts off. I am using an adjustable water pressure regulator and have it set at 60 PSI and have the standard shower head that came on my trailer it says it is 2.3 GPM flow. Sorry for making the post just a list of facts but I am trying to make it so that anyone that is interested will have all the info they need.  I could not find all of the info I needed so I took a bit of a leap of faith in purchasing and installing this thing.  The final result is that I now take a shower for as long as I want. For testing purposes I decided to take a 10 min shower and at the end of that I still had water just as hot as when I got in.  The water is hot enough I have to mix cold in to keep from burning me. The next shower I took I checked the temp of the hot water when I was done with my shower (about 5 min) and with no cold water it was 113 degrees on my infra red temp gun. I am extremely happy with this upgrade. I am currently living in my RV full time and have full hookups so grey water storage is a non issue, but even without full hookups it is really nice for me and my wife and two kids to take back to back showers without any waiting in between. I will try and get some pics up soon and by the way the excel unit shipped at 14 lbs so out of the box it probably weighs about 12 lbs. I also checked the temp of the fumes that come out the top of the unit when it is running and they are about 230 degrees as it comes out.  To light tissue paper you need more than 400 degrees so I feel very safe with it installed.

Some of the other posts I read on the topic were full of people who did not want to take a longer shower or claimed they could take as long a shower as they wanted without running out of hot water. That may be true in some instances, like in Arizona in the summer but as most of you know as you increase your altitude or as the input water temp decreases your shower gets shorter and shorter. For me when it is 6 AM and 40 degrees outside is just about the time I want a little longer hotter shower. When it is 95 degrees I don't mind a short cold shower, just my 2 cents.  I am happy to answer any questions anyone has on my install if you are interested.

Happy RVing, Ty

Here are a couple pics of the install.