To the original question; I would not and have not paid extra for an RV specific GPS. Indeed even the unit built into my coach is not RV specific as it does not have height information. Since I can get height information from the POI Factory and load them into my Garmin, that is what I did once.
In over 100K miles back and forth across the country I have seen at least a dozen well marked low clearances, and we spend most of our miles well off the interstates on blue highways. I even had to detour around one in a nearby town because I "knew" where I was was and forgot about it. Had enough advance warning BIG SIGN to modify my route through a neighborhood I had never seen.
We now use the onboard GPS for general routing and keep a smartphone handy with google nav ready to provide alternate directions when the first GPS guidance seems spooky. We use DeLorme Street Atlas to plan extended route Say Charlottesville VA to Austin TX and then build a daily route into the GPS for guidance and turn reminders. We do like the newer units that have lane guidance for getting through large cities. Traffic is nice to have, but in the coach a stop or a slowdown is really no issue and frequently the alternates are no fun.