The 3 stage units are really just trickle chargers on the lowest setting. They can still over-cook your batteries over time. IMO, the best option is to disconnect the batteries from the RV and then hook a Battery Minder to the batteries. A Battery Minder is a smart charger and has anti-sulfation techology. My tractor, RV, motorcycle batteries last 7 to 10 years using these devices and they are always topped off and ready to go.
2012 F150 FX4 Ecoboost, 2016 Surveyor 274BHS
2003 Yamaha FJR1300, Demco Premiere Slider
1969 John Deere 1020, 1940 Ford 9N, 1948 Ford 8N
Jonsered 535, Can of WD-40, Duct Tape
Red Green coffee mugs