The last two campers we have owned were bought new. I never experienced an odor of anything burning off for more than a minute or two. When we fire up the furnace for the first time, we generally have a smell of burning dust and other things in the duct work. It lasts a minute at most and then there is no odor again until the furnace sits for a long time without being run. Yes, the odor is horrible, awful, stinky, etc., but it doesn't last more than a minute.
Since the OP has been running his/her furnance for the last 2 weeks, I don't really think anything in the furnace is still burning up. There is a good possibility that something got in the duct work and died and that is what you smell. The air doesn't really get hot enough in the duct work to burn anything off.
If your vents aren't in the floor, it shouldn't be too hard to trace the duct work. I would start at the furnace and run a drier vent snake through it and see what comes out the other end. The ducting isn't super sturdy, so don't push the snake through too hard. You can also tie a piece of sturdy twine/rope to the furnace end of the ducting and pull it back to the vent, then empty the ducting. Then pull it back through to the furnace with the rope.
-Michael
