A month is about average for a connected battery to a travel trailer or 5th wheel. It only takes a couple of seconds to disconnect the negative lead when you aren't going to be using the trailer for a while. Battery should maintain at least 75% charge for three or four months when not left in service.
As for your "room" problem, get a little creative. Mine only had room for one 12 volt also so I turned them 90 degrees and filled in the cavity in the tongue and put two Trojans in side by side with no problem.
Good luck / Skip
