If the OP will invest the time and effort into providing the DC-DC charger with the proper sized wires, it won’t be just a “little bit of charging”. It will be 20 or 40 amps of multi-stage lithium optimized charging. I have a Redarc 25 amp DC-DC charger in my TC, and I think it’s great. I installed it according to the mfgr’s specs, including the recommended wire size for my circuit length.
OP, don’t use the tow vehicle or trailer wiring or the 7-way plug for this. You will probably need wires of about 4 awg, and either an Andersen power pole connector or a Pollak 2-pin lift gate plug. The DC-DC charger should be installed as close to the trailer batteries as possible. The wires on the tow vehicle side should connect to the TV batteries, not the alternator. I recommend using Temco fine strand welding cable for this.
:):)
