There have been a lot of GPS posts in the last few months and I think that the Garmin 770 has been favored by the RVing GPS crowd. More and more members have, however, admitted that any Garmin coupled with good paper maps are really as good as you can get.
If you plan your entire or even a good portion of your trips down to the grades you will encounter, a 770 is probably what you should look into.
The DW and I plan using paper maps and then change our plans as we learn during our travels about POIs we are interested in visiting. This usually happens in a campground or even talking to others while at some POI we wanted to see.
Option, you gotta have em!