Look for:
Water stains on floors walls, and ceiling (look in all cabinets and closets)
Discolored vinyl flooring
Soft spots in the floor
If it has laminated sides, look for waves in the surface.
On the outside look for cracked or missing trim pieces with rusted screw heads underneath.
All of the above, are indicators of poosible leaks.
Do an operational check of all systems.
Ask the age of tires and date of last packing or replacement of the wheel
bearings.
Ask if they have loaded weights on the trailer (gross and tongue weight), and make sure your tow vehicle is rated to pull it.
At the age of that unit, it really doesn't matter who the manufacturer was. The key to it's condition is how previous owners took care of it.
18 Nissan Titan XD
12 Flagstaff 831FKBSS
Wife and I
Retired Navy Master Chief (retired since 1995)