If you are going to use the Garmin with the RV in addition to in a car and you care about being routed around low overpasses, then get the Garmin RV760 or the truck version - Garmin Dezl 760. Both have RV mode and car mode. They also have the more advanced features of the upper car only Garmins. Both have lifetime maps and lifetime traffic.
If you don't care about RV routing then any of the Garmins are good - look for the features that you like in the different models. Costco has some good deals on Garmin car GPS stand alones.