Charging tent trailer battery directly thru car electrical outlet.

i just bougth a used 2006 viking epic tent trailer.i got the brake controller and ligths installed/working. But the guy from Uhaul did not run a live wire for accessories or line to charge the tent trailer battery as they are not allowed to do it.
Is it possible for me to use the 12 volt outlet of my truck directly connected to the positive and negative terminal of tent trailer battery to charge it or use it.

I would not suggest it, your 12 volt outlet is only good for 10 - 15 amps, a battery could draw more, and burn the wires.

One way is to wire a relay, and the power to turn on the relay coming from the ignition so when the ignition is off, it doesnt drain your truck battery.
And an addition switch to interrupt the power coming from the ignition so u can turn it off when you dont need a live power at the reat of the truck.

An old question I know, but one could also run the alternator's output through an isolator and use one of the branches to charge the trailer battery and tie the truck's battery into the other side.

An underhood switch to turn off the trailer circuit will prevent that plug from being hot while driving around town.

That's how multiple banks are wired in boats and RVs.