We own a 2104 Denali 330 RLS fifth wheel we purchased new.
A smell has gotten worse with time inside when using bath fan and sometimes when just using the kitchen sink. We finally tracked it down to the $5 air vent valve in the island cabinet and changed it out to a $25 'studor' valve from home depot. Then found the laundry drain pipe in the bedroom cabinet that only has a plastic cap loosely slide on it and replaced it with a rubber cap with hose clamp also from HD. Wow! fresh air again!
I hope this helps others with same issues!