I don't have a GPS unit stand alone, rather, I have a Garmin GPS software with a receiver which is installed on My Dell Netbook. The Netbook is the original 9" screen. This was back several years ago when better GPS units were still pricey. Not sure that is still the case.
I have this setup because 1) it was cost effective and 2) it gave me a larger screen size.
If you do a search on line you can find this kind of setup for trucks and RV's that cater to bridge weights and heights. I purchased my software from Staples. There are others besides Garmin. The Garmin I have is just a normal GPS.
gps navigation software for laptop - Bing
gps navigation software for truckers and RV's - Bing
I purchased a T-2053 just recently and a concern was a low overpass near the seller. Luckily I was going south, thus avoiding a 5 mile work-around detour.
That overpass has claimed a number of travel trailers!
One can't always be on the highway. Your question is a very valid one I feel. I also may be looking to update my GPS software soon.