I'm confused - electrical basics for simple system

[VanillaGorilla]

I'm almost done with my cargo trailer conversion, but the electrical stuff is putting up a roadblock.

I have installed 3 LED interior lights on the ceiling - those, and the outside running/brake lights are the only things running from 12v power. Right now, I just use the truck battery. Is there a simple diagram or system available to connect a dedicated battery on the trailer tongue to the 12v circuit, so that I can run those interior lights if the tow vehicle is disconnected from the trailer? Will this somehow interfere with the truck battery when the when the truck ign is on?

I have a generator that I will use for any (110? AC? don't know the right term) appliances I want to use inside the trailer, as well as for tire warmers (trailer is used for motorcycle racing). My trailer came with a simple exterior inlet box on the outside direct wired to a standard GFCI receptacle in the inside. Is this adequate/safe? I was planning on recharging the battery with a simple trickle charger from the generator as needed. I dislike the idea of adding wiring to the vehicle.

[parnold]

You will need to isolate your interior lights from your brake lights, or else your brake lights will be on every time you hook up your battery. The internal lights should be completely separate, then you have no issues with the tow vehicle battery.

Some people do have a plug, that they can connect the tow vehicle to the trailer battery for charging purposes. That would be the only reason to have the two connected, and you wouldn't want that connection while parked.. only while driving (or charging while parked).

[VanillaGorilla]

Yeah, I had just thought of that. Looks like I should have started with wiring, instead of recovering all the walls and doing cabinets first! D'oh!

Now I guess I'm going to have to go inside the loom from the outside and try to find out which is which that way. Ughhh....

[parnold]

If you have access to the wires at some point forward you'll need to disconnect them all to isolate the circuits. If you spliced wiring along the way, you'll have a dog of a time.

A small lantern battery and some alligator clips make a useful tool for tracking that stuff.