[Carl n Susan]

It really helps if you tell us the Year and model of your Montana. But in this case, if your RV is newer than 2010 or so, you have a "smart" charger converter installed. It will do a better job than a battery minder. So no, you don't need, or want, a separate battery minder.

Use your 50 reducer plug, turn of heavy use 120V appliance as Bob suggested, leave the converter on, do not disconnect the batteries, and use your rodent protectors. You will be fine.