[JRTJH]

If there's no air at any duct, I'd check the plenum box attached to the furnace. You should see a couple of 3" aluminum "flex ducts" connected and going to wall vents (if you have any) and maybe a 2" flex duct headed under the floor. The bottom of the plenum is probably positioned over a cutout in the floor and connects to the "central under-floor duct" that supplies warm air to the floor vents. There may or may not be an obstruction in that duct run. You can remove a vent cover, use a mirror and flash light to inspect inside the duct. You can also use a cell phone to take photos of what's under the floor.

Essentially, don't expect "house performance" from an RV furnace duct system. Most are "multiple runs of dryer vent ducting" with unfiltered air return and a furnace "efficiency rating" of around 50%.

What I'm suggesting is that you can do some checking to see if you can find an obvious problem, but taking it to the dealer will probably be a frustrating and not profitable venture. If the trailer stays relatively warm, they're probably going to tell you, "That's the best you can get" and give it back to you, hopefully in the same condition without any "new problems or damage"...