I have a deep cycle 12v battery in the teardrop and hooked up a 110v -> 12v trickle charger to charge up while connected to "shore" power. OK for short trips but on our Mexico trip we were going to be gone longer than the battery charge would hold out. Sooooo.....
I acquired a 400w inverter to run my laptop, and boom box that does 12v, 120v, pumps up tires and jump starts engines. It has the ability to be charged either with a 110v connection or a 12v connection. Soooo......
I ran 10 gauge red/black (fused, of course) from the alternator input of the fuse block of my truck, ran them into the back of the truck box (just inside the tailgate) and hooked them into a 12v power adapter - you know - one of those cigarette lighter things. So......
Now I can plug the inverter into the truck's 12v and let the alternator charge the inverter while driving down the road. BTW, it seems to take a couple of hours or less to recharge the inverter. Then....
I figured I could do the same for the 12v battery in the teardrop so I wired up a cigarette plugin on a pigtale, matched up the red/black and wired them into the battery in the teardrop in parellel with the trickle charger. NOW.... when the teardrop needs topped off, I just plug into the source in the truck and, after an hour or so of driving, the 12v battery charged up (assuming that I can trust me voltage regulator dial in the cab).
I also have a 110v extension cord in the teardrop that I use when 110v is available. In Mexico, however, the electricity is dicey and is upward of 130v in places. So, charging it off the truck alternator was a good alternative to have.
Works for me!
One word of caution - the cigarette plugin on the teardrop is "hot" since it is wired directly to the "+" and "-" posts and can short out and drain the battery. I made an insulator out of plastic 1/2" pipe that the plug fits into to prevent that.