I've owned my truck for right about 2 years... washed and waxed it for the first time this spring when the pollen finally died down.
I've owned my trailer for 2.5 years, never washed it. Its stored under a carport, but there are no sides so it gets some dirt. We clean out the inside every couple of trips, always after a trip to the beach with all that sand.
I'm just not someone who labors over a toy. I bought the truck to be a tool to tow my trailer. I bought my trailer to be a toy that is enjoyed and used. Its not nearly as fun to me to spend an entire day sweating and buffing a truck and trailer that will just get road grime and dirt the next day when I leave for a trip.
My friend, on the other hand... he washes and waxes his truck and camper before every trip. I joked to him over the 4th that he is gonna eventually wash the paint off his truck...