Received the rubber power cord/door thingy and it fits well. As long as they don't chew through the rubber it should keep them out, at least from that entrance. Once we discovered the initial problem and put steel wool in the power cord door we did not have further mice problems, so I hope this addresses the issue.
The trailer is located on our farm acreage and because we don't live there yet having a cat isn't going to work.
I rolled around under the tt looking for possible entry points but it looks pretty tight under there. There is some sort of fabric underbelly covering the bottom but of course there are some penetrations such as the propane pipe, a couple of water drains, and a couple of electrical cables. (the tanks are below the floor/fabric)
I'm a bit baffled on the mice getting up on top of the propane tanks under the hard plastic cover. I assume that once they get to the propane tank platform they are using the rubber hose to get to the top of the tanks, or maybe the threaded hold down rod?
If the mice can get into the frame rail (c-channel) they could run front to back but at the front there is the end cross rail and the tongue A frame is welded to that. (propane tanks are on the tongue) It seems it would take some serious gymnastics to get into the A frame channel from main channels but I guess one should never underestimate these rodents.
The tongue is 20" off the ground and I hook the safety chains back to the equalizer brackets on each side of the tongue. The chain droops down to 14" off the ground.
Think it's possible they are jumping 20" up to the tongue or maybe getting on the safety chain, or possibly somehow scaling the actual tongue jack up to the propane tank platform?