The furnace on an RV can be extremely noisy/loud. Neighbor came in and parked next to us; first RV - new Cougar 1/2 ton 5th wheel. First morning he was out pulling off the cover to his furnace. I asked what was wrong - it's TOO loud; something must be wrong. Had him fire it up and it sounds like any other 35k RV furnace, which I told him. I also recommended he take a walk around the park just before sunrise and listen to the cacophony of noise as the furnaces kick off and on around the park then come back and let me know. 2-3 days later I asked if he had made that walk and he had. Said he could not believe how loud all those things were kicking on and off. Hasn't said a workd since. He did however buy a little Lasco tower heater to help with the noise and LP consumption.
Return air at the foot of the bed? Sounds like you have some built in nighttime noise so "other" things don't bother you. Our last trailer's return air was right at the end of the kitchen. Sounded like a jet when it was running. Tore into it checking for anything to reduce the noise...nothing. It is what it is. Just added speakers to the TV and turned it up when the furnace or the AC was running....
It is what it is pretty much.
I would take a good hard look, and think long and hard, before I took it in, let it sit for months at the dealer just to have them come back and tell you that it's normal.