Do you mean like this?:
In place of that small panel, look at whay they call an "all in one."
Another good choice is to purchase a pre-made pedestal; you might even save some money.
Anyway, the pressure-treated post gets set 3ft. down, and a little concrete doesn't hurt. A larger panel would probably benefit from a bacjing board. make sure all holes are plugged, and that you maintain the space between the post and the panel.
You need to pipe to a spot under the trailer and set a junction box; make sure the trailer cable can reach! The pipe needs to be 24" down - there's an additional HUD standard, in addition to the NEC.
What size service? Look for the nametag on the trailer. Treat the trailer as if it were a big toaster - not like it were a home.
You also need a ground rod. If your trench is 30" deep, you can lay the rod in the trench.