..The best answer to this issue, came in the first reply/post - Dutch's comments about a charge line to the MH.
A solar panel may well work, too, but it sure seems like spending a lot of $$ unnecessarily and making a simple problem much more complicated than it needs to be.
Go get you some wire, connectors, fuse links, etc. for less than $20, and wire you a charge line from MH to the towed vehicle. Problem solved. Batteries stay charged up all the time when towing, no matter what drains there may be on the battery. Very simple, quick and easy to set up once, and you'll never have to worry about it again.