Three options one may not apply.
First.. The one that may not: ON SOME Rv's the converter is a plug in model, this is both a blessing and a curse cause if it is.. It is the answer to your question. (Plug just the converter into an extension cord..NOTE you may need a modified extension cord (I DO, Mine is I have the proper adapter).
Second: Add a battery minder or battery tender and plug it in when at home. (or a small, (Smallest you can get) QuALITY 3 stage converter)
First: Get a small can or bottle of paint of finger polish (1 OZ is more than enough color RED (you may also wish to get a 2nd one BLACK)
Find the battery terminal marked with the + (POS) symbol or letters that has wires running off to who knows where (NOT to another battery)
PAint all wires and the area on the battery near the terminal RED (Spray works well if you like)
Optional black goes on the NEGATIVE terminal that runs off to who knows where
Now take a photo with your phone
Remove batteries to bench and use the battery minder there
NOTE: Always set batteries on a sacrifical surface like a piece of scrap plywood or a few layers of corograted paper.. NO this has nothing to do wiht protecting the batteries. but if there is any acid leakage.. Scrap lumber costs way less than whatever is below it.
Home is where I park it!