inside trailer wireing

Depending on the service you are planning on using you can put a standard 30 amp plug from any DIY store. I built a long extension cord for my camper with a 30 amp service and picked up everything from Lowes. If you go to a camp ground that has 50 amp service all you need is the adapter, which can be picked up at any camper store.

BTW, if you haven't bought your cord yet, purchase a 10/3 wire, the next size up won't fit in the plug. The 10/3 wire will be good for the 30 amps with no issues.

Like Pops Said, the two standard RV Plugs are 30 or 50 amps. However, when your buying the plugs, make sure they are 120 volt, not 240 volt plugs. It will be very common to carry an adapter to convert the 30 amp gender to a 50 amp gender and vice versa. You will also likely want to pick up an adapter that will change the plug to a 15/20 amp standard household plug too. This comes in Handy for doing general stuff that just requires the interior lighting, and in some cases may also run the AC unit, depending on the current draw.

I had an array of adapters for my motorhome that worked out very well.

You could consider wiring it up like you find on a pop-up camper. Build a small storage box in the nose & coil a long cord inside a compartment within the box, feeding through a hole in the side of the trailer. Make it long enough to reach the utility post in most cases & carry another extension for the really long runs.
You can get the plastic feed-thru & cover that hides the plug at a camper parts place.

thanks, those are some good ideas