I use a Garmin RV-660LMT. It has a car mode which functions like your Nuvi 50 and routes you down the shortest/quickest "car driveable" roads. Which is great when you're NOT towing/driving your RV. It also has an RV mode which will route you down more commercial grade roads that can handle your specific RV requirements (weight, height, length, road grade, etc). You configure the GPS to your specific RV measurements and it finds the best roads.
I got my RV-660 after my old GPS routed me down a narrow dirt road that had an old wooden bridge with a 10'4" clearance and 6,000# weight limit. My TT requires 10'6" and 7,000#. DW was to NOT happy walking behind the TT as I backed-up for 1/2 mi until I found a wide enough driveway to turn around my 30' TT.