I have a couple of 220V tool outlets I installed in my garage, that are 3 conductor 240V with a ground. While you can get 120V to ground off one side, it is a safety hazard because the ground conductor is conducting the return current, and it can create a significant voltage difference between the safety ground of your trailer and the actual ground, which isn't protected by a GFCI or circuit breaker. If the outlet is a 4 wire receptacle that is properly grounded, like one for an electric range, then you are ok after you confirm that it is properly wired. (4 wires all the way back to the panel, ground only bonded at panel). I don't trust anything 240 volts that was wired by someone else, even residential electricians. I'll check with a meter, and if necessary I'll pull off the cover around the outlet and visually inspect the wiring.
I nearly seriously injured myself using a 240V tablesaw in my garage after I had first moved there. The saw and outlet were wired "properly", but the panel wasn't. The panel was a subpanel and the ground was bonded to the neutral, which is not code in my area, it is only supposed to be bonded at the main panel. The neutral between the subpanel and the main had gotten fried, and I touched the metal table while pushing a board through the saw and got a serious shock. I almost put my arm through the blade. I took a DMM reading between the table and a nearby metal object and got over 100V.
