cleaning duct work

Very carefully. If you knock it loose somewhere underneath, then you have a job taking the underbelly off to get at it to put it back together. For the most part trailers use the same mylar flexible ducting as a residential home does so it is fairly tough and can stand some abuse while cleaning. The way they put trailers together though I would be concerned about other damage.

As far as the duct cleaning business goes, I worked in HVAC before and attest to the validity of it. A friend just had it done in his older 70's home and they took out 7 lbs. of dirt. Yeah, it builds up.