You may keep your trailer at your site, but it must fit on your site either on the driveway or sand pad.
Personally, I would just park it on the main lot to get it out of the way. With camper, tow vehicle, picnic table and golf cart, I wouldn't want to crowd up the site with a trailer also.
Last time we tent camped we had a small utility trailer and we had shoved it off to one side and received a note on my windshield to move the trailer onto the site or out to the main parking lot. I moved it onto the site for one night, but soon realized it made everything much more crowded, so I moved it to the main lot the next morning. I just locked it, and taped a WDW parking permit to it.
Carol