Hey Toy,
Like everything in life this can be simple or complex depending on how you want it to look. The simplist solution is to mount your inverter somewhere in the trailer that has easy access to 12V power. Depending on the size of inverter you plan on using you should probably run 10 gauge wire froim your battery to the inverter. You should also put a inline fuse in this power feed to ensure you are protected from a short. 10 gauge wire is good for 30 amps so a 30 amp fuse or less is what you want. Most of the inverters already have a 110 outlet on the side of them that you can plug your extension cord into. About as simple as it gets.
You could also wire up a set of outlets in your trailer and power them with your inverter instead of using the extension cord. Good luck, Jerry.