It may be your battery is getting old, However you have cited ONE of the three reasons I do not like those type of systems..
There is a system called "Towed Charge" or some such, basically it adds one more wire to your interconnect, this is connected to either an isolator up in the motor home or to a circuit breaker at the motor home end, Gives the towing vehicle (motor home) the ability to charge the towed.
Oh, the other reasons I don't like them?
You have to Install every time you tow
And "Oh, it's only a short trip, It is too much trouble to wrestle this thing in and out for just the few miles I'm goin... CRASH, Dang I wish I'd installed it"
Home is where I park it!