Hooking up a 12v battery a couple of ???

That is what I did. The problem is the guage of the wire will not charge the battery as well as you would like. I have seen a lot of truck/trailer combos here that have an additional large guage cable/plug between the truck and trailer. This goes direct to the battery of the tow vehicle. Make sure that you unplug it or it will drain your tow vehicle. Whenever my trailer is parked at my house, I have a battery charger hooked up to my trailer battery so that it is 100% charged when I leave.

As I see it the "Charge Wire" in the trailer plug is only designed to keep the trailer batter topped off. There is no way the vehicle manufacture could ever know what kind of load you would be adding . The wire running from the truck should be fused as close as possible to the trucks battery, if your adding this wire I think I would use a minimum of a 10guage wire and a 20amp fuse , the heavy duty plugs are able to handle 40amps when new and we all know how much care is given to a trailer plug.
The battery in the trailer should also be fused just in case you get a short in the trailer. The battery wire can then run to a small fuse panel to run the lights, heat and stereo. The battery and all accessories should be grounded together on the frame. ( my reasoning is moisture and rust is always a problem on trailers) When the trailer is at home of parked I use a small charger/maintainer to keep the battery topped off. They make several types that can be permanently mounted in the trailer.
To keep the truck from getting a dead battery you should put a constant duty relay under the hood of the truck that will only allow current to flow when the trucks ignition is on .

Wiring Diagrams???

I am also looking at placing a battery inside my trailer. Does anyone have a picture or a simple diagram showing the connection between the trailers wiring and the battery in side the trailer. Also, if anyone can explain how to setup a relay to prevent damaging the truck's electrical system if there were a short in the system. What brand/model are you using for a permanently mounted charger?

Thanks for any advise!

On the truck end you need a relay that will only alow the trailer to charge when the truck is running and a fuse under the hood to protect the truck and charge wire. On the trailer end you need a fuse at the battery for the trailer and each item that you are going to run . As for a charger/tender look at a boat shop or Cabelas (sp) , I think they are about $80 for a single battery and go up from there.

I was a RV tech for 9 yrs and here is what I would do .
1. Don't rewire your truck they most come from the factory prewired to charge your battery. They are not like snowmobile manufactures it works .If you want to put a isolator in the chargeline that is ok some come that way .
2. At the tongue of your trailer there should be a box where the plug wires go into . Open it up take the big black wire which should be the hot wire and run a wire from the positive side of the battery . Then the big white wire should be the ground run a wire from the ground side of your battery .
3. Then find where the interior lights hook into your clearance lights and unhook it and run a new wires to the interior lights from your battery .
4. A battery tender for your trailer is a good idea

Electrical Plan...

I think I have figured out how to wire in a battery to the trailer. I plan on putting in a charger/maintainer along with an inverter to run some 110 lights and add a couple of receptacles. My questions are:

If I were to plug in a male to male extension cord from say a generator into one of the receptacles for power while camping would that fry the inverter or battery? Or would you just plug in the charger/maintainer to the generator and then increase the amps the charger is putting out to the battery?

To wire in the lights and receptacles I was planning on wiring in a plug to romex that would plug into the inverter and then create a run for the lights and a seperate run for the receptacles. Also, I was planning on wiring in a breaker between the runs and the inverter.

Does this plan sound right? Any concerns or words of caution...Aslo if anyone has a recommendation on the type/brand of battery, maintainer, inverter that would be helpful!