We had pop-ups and then two travel trailers over a 25 year time span. Based on my experiences:
1. Get new tires and go with one of the better brands such as the Good Year Endurance or Maxxis tires (Maxxis tires worked well for me).
2. I had brakes checked and serviced at least every other year, and would definitely have the checked/serviced before a high mileage road trip.
3. Check all seams yearly and seal as necessary. It's usually a good time to do it when you are giving the roof a good cleaning, checking AC units, etc.
4. Check and clean out the access areas for the water heater and refrigerator yearly. I had forgot about checking the refrigerator access panel on our MH for quite some time, and I recently opened it up and it definitely needed cleaning....quite a bit of dirt and dust. Visually check the gas flame to make sure it looks like it is burning properly. Watch out for spiders building webs in the refrigerator flue pipe (I think that's the right term). Can brush it out with a small brush...it may require removing a metal shield.
5. Check under the camper to make sure all wiring is properly attached (not dragging, hanging down, etc.). If the underside is covered, look and feel for any indication of water pooling or collecting.
Hope that helps!