I think that needing an RV or truck specific GPS depends on a lot of things. West of the rockies there are really not all that many low clearance bridges and underpasses. But if I were traveling the east coast - and especially New England - in a big rig I'd sure want all the help I could get finding tight spots and avoiding toll roads and heavy traffic. As far as I'm concerned, anything east of Chicago has heavy traffic!
The points of interest on a truck GPS are probably a lot different than those for an RV. Having said that I have to admit that most POIs I look for in whatever GPS I have aren't there... so, at least for me, POIs aren't that big.
I like GPS units for a variety of reasons and almost always have one on when we're traveling; even by car. Partly to keep me in the right lane at the right time in traffic. But I often check my arrival time, look at elevation, check my "readl" speed, etc.
Of course, you need to use every appropriate guide and not just a GPS. But everyone knows that by now, right? :P
WDR