Top on my list would be: window reseal (butyl tape), body to frame bolts, wheel bearings, and frame. If
tires are old (they will have date codes on them) replace them too. Trailer
tires are relatively cheap. Not worth the risk of trying to get another year out of them. I'd replace the leaf springs too.
When I bought my
Trillium last year, the
tires were either 24 or 34 years old! (Tire date codes were not as clear back then). Fortunately I had brought a set of replacement tires on wheels with me for the trip home.
Interior walls, particularly the vertical ones, are part of the structural strength of the trailer. Some of the horizontal cabinets provide structure too. Consider how thin the
fiberglass is.