Im on the fence with your concern. I understand that the phantom loads are draining ur battery while in extended storage, but I would just pull the 30A fuse on the tongue before leaving it...problem solved.
If you want to keep it "up and running" and put back more than the phantom loads take out then its going to take at least 25-40W of solar, wiring, charge controller etc to accomplish this. If its a portable system are you confident it wont be stolen?
Unless you want to install a somewhat robust solar system for true charge capabilities I would just install an inexpensive cut off switch and use it in storage. Those tiny/cheap solar "maintainers" are in my mind simply useless in rv's and a cloudy week may mean you still show up to a dead battery. Just my .02