With used, condition is everything. It doesn't matter if it's a great deal if there are problems. As you note, the biggest concern is with water intrusion.
Check all of the floor for soft spots. There will often be some give in the flooring but there shouldn't be much sinking. Look underneath the Aliner at the flooring for rot or soft spots. I use a key to just push lightly into the wood. If you get anything that flakes off easily, be done there unless you're into and enjoy rebuilding.
Look at the ceiling inside for any bubbling or discoloration. Check all appliances for function, same with water system.
On the roof, I haven't been able to find any historical information on the roof materials - just what they have out there currently, and I quote:
The sidewalls, the hinged “A” walls, and the front and rear roof are made from a composite material
known as foam core lamination. At our factory, we laminate 2.7 millimeter “superlite” paneling, 1”
polystyrene foam, and 1.30 millimeter fiberglass together with a urethane adhesive. The lamination is
then placed in a vacuum press. The resulting solid core product is lightweight, extremely strong, and
thermally insulated. This rigid 1 1/8” material also eliminates the weight and wall thickness of the stud
and bracing method. The exposed edges of the walls and roof are capped with anodized aluminum extrusions. Exterior flanges, covers, vents, and baggage doors are affixed to the outside walls with silicone and screws before finishing with a sealing bead of silicone around the perimeter.
It's nice that they tell you expressly what it is made of and how it is put together.
I'm no expert in inspecting fiberglass for damage and dents. Take a good hard look and then go with your gut, especially if others here can give some better guidelines.
If all checks out, I think you'll be quite happy with an Aliner. I have only heard of a few who had problems. All of the Aliner owners I actually know or am acquainted with are quite enamored with theirs. I can see why.
