I installed a relay in the void behind a tow vehicle tail light
I tapped into the tail light
feed wire and fastened the new lead to the relay coil.
The other side of the coil is fastened to ground.
The wire from the battery ( make sure it has a 30 amp fuse in line close to the battery ) goes to one of the relay contacts.
The other contact goes to the proper contact on the 7 pin plug.
Make sure both the hot wire and ground wires lead directly to the battery terminals in the trailer.
The feed wires from the tow battery to the camper battery are at least 10 guage. Bigger is always better.
Now when towing, the lights
are always on in the TV thus energizing the relay and thus the camper battery. This assures that you can not drain the TV battery unless you happen to leave the lights
on on the TV.
The regulator in the TV takes care of the charging of both batteries.
When you are plugged in to 120 volts, the converter takes care of charging the camper battery.