I have a Pollak 7-pin trailer plug that I wired to power the repeater from the +12VDC and ground connections. I plug it into the truck's unused receiver (bumper) receptacle when towing. Current draw is in milliamperes, so to draw down all the batteries in the truck and the RV while they are hitched together would take weeks or months. When I unhitch the 5th wheel, I unplug the repeater and store it away.
Another approach would be to wire it into a switched +12VDC source on the truck that's only powered when the ignition switch is turned on.
Rusty