I just purchased the Cobra Truckers GPS, i have not used it yet but it is highly regarded, G

The only thing you have to be concerned about in any way is height and that's not really that big a deal since just about any commercial truck is higher than any RV. In 4 years of traveling we have only once come across a low clearance problem and we don't even check for them. Vehicle widths are regulated, they are all 102 inches or less and all roads are built accordingly, so that's not an issue. As for length, there are tractor trailers that are 75 feet long all over the place. Bottom line is that your safe driving on just about any road in the country without issue, even on residential side streets. When we first get in these things we think they are HUGE and we won't fit anywhere, but when we finally pull into a truck stop an put it alongside of a commercial rig we realize how small they really are. It all becomes easy very quickly, just take a deep breath and go slow.

Not a Garmin, but I got the 2010 Motor Carriers Road Atlas at Barnes and Noble recently.  Has low clearances in it...probably not all of them...but it is pretty neat.

Garmin now only makes one GPS with large vehicle routing the 465T and it is over $300.  Here is
Amazon.com's page with reviews.

We use our Garman all the time and have not ever had a height problem. We can't live without or GPS...don't know how we ever got along without it???

I bought the Garmin 465T it gives you a truckers mode or car mode, you select the mode you want to use. The trucker mode will allow you to select ht, wt, and width. It will keep you off the roads you should not be on based on the data you input to it. I like Garmin, but as others have said you should always preplan with maps and I go to Mapquest and run the route also. I don't believe you can be to well prepared.

The Garmin 465T is the only one I am familiar with that allows you to input the information you speak of. I do not have any experience with the Cobra unit for truckers. I do not have one of those units but wish that I did. The other GPS's that I have used have gotten me into some situations that I do not care to be in with our 40 foot motor coach and toad. Height has not been so much of an issue but narrow streets, unpaved roads and narrow bridges with weight limits have been.

I use to have GPS navigation on my phone and the funny thing is that I would still use a map to plan my trip and just used the Navigation function of where I was at. I have since taken that function off my phone because it was two expensive.