RNG, Thanks.
Problem 1) To pull a wagon (horse or vehicle) on the streets here in FL it has to have headlights, tail lights, flashers. This wagon also has elect. brakes. All of this is no problem by adding a battery to run it all, my question is I have a set of shafts for the horse to pull it, and those can be switched out for a tow bar for the truck or tractor to pull it (like taking it to a parade). How do I connect a RV 7 Pin trailer plug to it with out getting feed back to the stand alone battery. and make everything work.
Problem 2) Wanting a brake pedal for the brakes, will a 120V AC variable switch (sewing machine pedal) work on 12V DC? I know I can do it with a trailer brake controller and just use the manual switch, but that doesn't give me a pedal for my foot while my hands are full of reins.
Bet you want to recant your offer now, huh? :laughing:
Nah. What you're wanting to do isn't too complicated if you take it a piece at a time. But first a question:
Will you be activating the turn signals, brake lights, and headlights when the wagon is horse drawn?
If so, then you'll need some kind of switch for each of those circuits. And when the wagon is being towed by a truck, those switches will need to be wired in parallel with the ones in the truck that control the same lamps. That means that the wire from the trailer connector on the wagon, which will have 12V on it when the corresponding switch in the truck is closed, needs to go to the switched side of the switch in the wagon, which is the same as the switched side of the lamp (the other side of all the lamps in the wagon will go to ground, and that's just the negative lead of the battery in the wagon).
The trailer connector on your truck may or may not have a hot lead in it that is either always on or switched on with the truck ignition. You could wire that lead to the positive terminal of the battery in the wagon and the battery would charge when the trailer connector is connected. I don't see any harm there, other than the possibility of leaving the trailer lights on and discharging both batteries, but that is only a possibility if the hot lead in the truck trailer connector is not switched with the truck ignition.
Then there's the ground lead from the truck, which should be wired to the negative terminal on the battery via the trailer connector.
As far as the trailer brake is concerned, I'm only familiar with the kind that use an electromagnet in the brake assembly to activate the brake shoe. The more current that is applied to the electromagnet, the stronger the brake force. The ones I've seen aren't very strong, certainly not strong enough to lock the trailer tires, even on gravel. As such the trailer brake is intended to help the main brakes, which are in the tow vehicle, and they aren't intended to be the only brake in the system. I don't know how much your wagon weighs or how much you intend to haul with it, but you might want to think about how fast you need to stop if you're going to use an electric brake as I've described.
As far as how to wire the electric trailer brake, if your sewing machine switch is nothing more than a variable resistor, it would work, as long as the resistance is very high when you have your foot off the pedal. It would be better if it also had a switch in it so that the first bit of pedal movement closes the switch. If not, there would always be some small amount of current going to the electromagnets in the trailer brakes, and in a total loss system like when you've got the horses hitched to the wagon, the battery will eventually go flat. Either way, one side of the sewing machine pedal would go to the battery on the wagon, the other to the electromagnets on the trailer brakes. And the brake lead in the trailer connector would also go to the trailer brake electromagnets.
Clear as mud? I can see the schematic in my mind, but I'm too lazy to draw it out for you. If I was doing the job myself that's the first thing I'd do though. If you see things the same way, have a go at a schematic, take a picture of it, and post it here. I'm sure all the electrical experts will have a field day with it. :laughing:
. As far as brakes. Good luck. Electric trailer brakes are the one thing I don't have the patience for. I've found that when 12 volt wiring stats giving you trouble it will until it's all tore out and replaced. LED is the way to go also. 
