hi 92' and welcome to the forums...
smells are tricky, can you post some of it here?
IF the odor is actually related to formaldehyde, the source is more likely to be from tank chemicals than trailer parts...
16 years should be long enough for fabrics, wood and other materials that might have contained formaldehyde...
and really there weren't than many materials inside a 90s unit unless it's been modified.
have you tried REALLY CLEANING all the tanks, vent tubes, and so on?
IF the previous owners regularly used formalyn based tank products, it could take a LOT of flushing...
and the toilet gasket/seal may need replaced too...
cheers
2air'