This is my biased opinion but in my experience Trilliums are a notch above Bolers. Expect Bolerites to respond otherwise.
No frame issues with the '77 models.
Door sag can happen on either Trills or Bolers but if the pics are of the one you are considering it looks fine.
Most parts should not be a problem.
Again, if the pic is the one, it looks to have excellent wheel clearance which should indicate a good axle
, perhaps already replaced. If not replacement axles of the same type are readily available.
The Trillium has a complete fibreglass shell on the bottom so the plywood floor is sealed from below and also fibreglassed on top on the inside. Should be a non issue.
Trilliums are a bit taller which translates to more headroom inside. Weight
difference is minimal and will tow just as easily.
The biggeset issue seems to be leakage around the windows
if the seal around the around the windows
has broken down. Check carefully inside along the bottom of the windows
, the upholstery, and in the hatches for signs if water infiltration. This looks like a very clean example and I would not be surprised if the windows have been recently resealed.
If the price is good go for it.