There is romex (and equivalents from other brands) that is rated for being buried without a conduit. The versions I've seen (as of a few years ago) had the outer "armor" layer colored gray and came in a number of gauges and conductor counts. There is very little premium on the price vs. indoor romex.
(Note that the color may be different now - there has been a move to color code at least indoor romex to indicate its gauge at a glance and that may have been extended to underground rated stuff, too.)
You'll probably want to - and may be required by code to - have a short section of conduit where it enters/leaves the ground. But that's also good to protect it from trouble, such as weed whips, UV from sunlight, and moisture infiltration at the end where the outer layer is open. A little hunk of treated 4x4 post with a couple feet of conduit and an outdoor box fastened to it, next to your trailer parking space, seems like a small cost. You'll want to put the "wires leaving the end of a conduit" cap on the underground end to avoid abrasion against the cut end of the conduit and (if convenient) have a 90 degree bend to start the romex in the right direction for its run. There are also outlet box covers available that are weather rated even when a cable is plugged in. You'll want one of those, too.
I don't recommend using buried copper pipe as a conductor. You'd have to insulate it - especially with it carrying DC - or you'd get KING HELL corrosion on the positive pipe from currents through the ground between the pipes. The cost of both wire and pipe is mainly the metal. By the time you got your pipes and their associated stuff installed, buried, and hooked up, you'd be out far more money than if you used underground-rated romex in the first place.
If you do find aluminum wire for your run, be sure to use the proper aluminum-to-copper joints at the ends. Aluminum wire is a fire hazard (due to the joints becoming resistive with age) and the heating at bad joints is an issue with CURRENT, not voltage, so the fact that you're running less than 120V makes it worse, not better. A fire in a trailer is more dangerous than one in a house, too. (On the other hand, a properly terminated aluminum wire run could save a bunch of bux without degrading the instalation's performance or appreciably increasing the fire risk.)