Hi there. I'm just a maker with no formal training in electronics, but understand most of the basics.

New to me camper has a 12V battery to power lights and a fan onboard. It also powers one high power device which is the tongue jack, and also activates the brakes on the trailer in the event the trailer breaks away from the tow vehicle.

there is a solar panel with a renogy charge controller installed. Currently there is a switch to connect charge controller to the battery. PO told me to be sure I had that switch off if I plugged in the camper, or plug the towing connector into the vehicle. (When connected to the vehicle there is a12V+ connection that charges the battery.) PO indicated that power from other sources backfeeding into that charge controller would damage it and that makes sense to me.

My initial thought was a SPDT relay that would have the NC contacts connecting the solar charge controller to the battery. If mains or vehicle power was present the relay would pull in and connect that instead. But once that happened the battery would be connected to the relay coil and it would not switch back to solar when that source was removed. Would just a diode work somehow? I'm thinking there could be considerable current in certain situations. I also don't want any extra resistance in the circuit when solar is charging the battery.

Thanks!

 

That's a tricky problem.
The easiest is to put a large diode in series with the battery connection from the vehicle, but the diode drop will adversely affect the charging of the trailer battery.

An alternate way would be to use a voltage sensitive relay circuit to control crossover.
The relay activates when it detects the vehicle engine is running and the battery voltage is above the charging value.
So you should be able to use the output of one like this to control your changeover relay.
(Note, that the output of the module will only be connected to the coil of your relay, and not the trailer (house) battery as shown in the connection diagram).