The RV 760 LMT looks like a nice unit. It would have to be pretty much an "in truck" as it only has 1.5 hour battery life.
The III Plus was a nice unit, but it can't "auto route". Any route you built was simply a collection of straight lines between waypoint. Used mine for a number of years and then moved on up to the V and then the Quest, then the big jump to the Zumo 660.
But using MapSource and then BaseCamp have always been the key to success for me. Got me to spend the time to plan the trip, figure out where to get fuel, where to eat, where to stay and the route to follow.
So in the end, my GPS sort of wound up being a reference to look at on the dash...I had the whole trip in my head.
That's been my issue with using the GPS to enter start and finish addresses, or like the GPS in wife's Tacoma...you have to enter addresses, not coordinates...and then it decides how to get there. Ummm...NO!!
Just a few more thoughts,