I have always found Garmin Customer Service to be among the best out there. Call them and describe your needs. They will most likely have a range of unit to pick from.
This may help.
RV GPS Comparison and Review: What are the Best Options?
Personally I can't see any advantage in sticking to an RV GPS with an Airstream. You don't have the weight issue, height issue, or limited access at fuel stops to deal with. I find the most useful function on my several different GPSs is the one that allows me to mark on the screen, and identify with a unique symbol, points of interest for future interest. That neat sandwich shop we found on an unnamed crossroads and the different campgrounds we frequent.
You cab go wild with requirements, traffic, blue tooth, voice activated, several different turn display options so make a list. Another consideration is how soon before and how ofter will it give notification of a turn. This is important when dealing with interstates. Notification to a turn after you have passed the ramp is not uncommon with some units because they consider the actual crossing of the roads as the point of the turn.