This could get long but I'll talk about what you need...you can make it as simple as you want or as complex...suggest you refer to phrannie.org for a lot more info.

First for the 110volt ac..you will need to mount a small circuit breaker box in your trailer.  Say a 6 circuit box available from lowes or home depot. Inside the box you  should have a ground lug that will connect all your grounds from the 110vac outlets, lights, etc to the ground/trailer frame and the green wire that connects to the main ac plug/cable.  Also there should be a neutral lug inside the box that all the neutrals (white) wires from your outlets/lights etc connect to and also the white wire to the main ac cable...lastly the main ac cable black or hot wire will connect to the bussbar inside the box that the breakers are connected ...then the hot wires (black) that connect to your 110vac outlets are connected to the circuit breakers...likewise if you have a roofmount ac unit, it would be connected to its dedicated breaker.  Some folks wire up a direct extension cord to the breaker box for use as a main plug..others put in a bulkhead connector and then use a rv type extension cord.  Look at some factory trailers/rvs for how its done. 

For the 12volts, most trailers have a 12volt battery mounted on the trailer and use 12volt lights extensively...buy these from any rv supply store or west marine for the better quality.  You can use 12volt rated switches at an autoparts store...most simply use 110volt rated light switches to control the lights..Run a heavy battery cable from the - (neg) post to the trailer frame.  The + (positive) post will connect to your light switches and then to the lights using at least 14awg stranded wiring.  You can use crimp connectors for the 12volt wiring.  You can also purchase a small fuse panel or breaker panel for the 12volt wiring from most rv supply stores or west marine.

Past this you can install a converter that supplies 12 volts to charge your battery and provide 12volts to your lights when you are plugged into 120volt ac...these converters are available from rv supply stores.  Then there are inverters that provide 110vac from your 12volt battery..then there are solar panels...then generators...goes on and on depending on how much $$ you want to spend.

Suggest you visit some horse trailer dealers in your area and look at  some that are finished out...most basic unfinished trailers have at least a 110volt ac breaker box installed.

Hope this helps.

Thanks Todd, I really like your LQ. Just a little apprehensive about DIY stuff.