Just bought a 2007 Funfinder X139. When I was under the trailer while putting it up for the winter, I looked for places where mice could enter. I found that a few of the holes where wiring enters the trailer floor were filled with foam. A good idea, except that in one or two places the foam had fallen off leaving a hole where mice could enter the trailer. I plugged them with a silicone caulk and plan to inspect the underfloor each year to prevent rodent entry.
I also noticed that the holes where the wiring passed through the frame didn't have a spacer or grommet. So the wiring is free to move about, rubbing the insulation off during transit. This could cause an electrical short or a blown fuse when the bare wire contacts the frame sometime in the future. I would suggest that it would be easy to get the proper size plastic wire wrap, or shrink wrap from any auto supply store and wrap the wire where it passes through the metal frame. The correct size will fit tight in the hole, looks like plastic pipe wrap with a slit on one side to pass the wires into. Cut off a 2 inch piece, put the wires inside through the slit, and then slide it into the frame hole. If you wish, electrical tape will close the slit and it will fit tightly enough in the frame hole to stay in place. That will keep the insulation from wearing out due to vibration during travel. It only takes a few minutes, costs very little. I think a foot of the stuff might run you $1 or so. But oh the aggravation it will save when you encounter problems with your tail lights, or brake lights when you are on the road. I did this job in about an hour, learned the hard way with my boat trailer over the years. Just a suggestion, if you need help understanding, I can take a few pics for you if you email me.
Another thing I found is that the spare tire holder didn't look like it had any caulking on it. I removed it and found it was completely dry, an easy place for water to enter a drip at a time and bingo, it would be in your wall and on the floor, you would probably notice it with the vinyl flooring coming loose from the plywood. Another easy job, remove it, caulk it up good, reinstall it and you will have saved yourself some grief in the future.
These are just a few things I found on my X139 during my first winterizing experience. Hope it can save you some grief in time!
2007 Funfinder X139
2004 GMC 1500 PU crew cab 4WD