If you have a 120VAC RECEPTACLE close to where the trailer is parked to be able to plug in a trickle charger why not just pick up a 30A-15A long Adapter cable from WALMART and plug your Trailer 30A Sshore Power cable into the available 120VAC receptacle. Then the on-board converter/charger will keep your battery charged up.
This is what we do here for our 5th wheel parked in the back yard...
In my case my parked trailers are some 60 feet away from my garage 120VAC receptacle so I use a good quality 50-foot Contractor type 10-3 extension cord and then plug my trailer into that. A 12-3 extension cord will work as well but anything smaller than 12GA is not a good idea... We can actually run just about everything in the 5th wheel and/or the OFF-ROAD POPUP trailer we have (Including the Air Conditioner or Microwave). The deal is we have to watch what is on at the same time otherwise it will trip the house circuit breaker for the garage 120VAC receptacle...
We spend alot of time in both of our trailers sitting in the back yard.. A great get-away spot at home...
Roy Ken