The best way to clean RV heat duct work is to use a standard plumber's tool. You can access all the places you'll need to, and your ducts will be clean and virtually as good as new. You will have no harsh chemicals and no water to worry about--just good old-fashioned elbow grease will get that RV heat duct work clean with not much hassle at all.
Items you will need
Flat-blade screwdriver 50-foot-long plumbers' pipe snake Large old rag Duct tape
Pull the duct off the furnace. At the RV furnace base, there will be a clamp or two holding on the actual heating duct. This clamp will be held on by a screw or two, so remove those screws with your flat-blade screwdriver.
Wrap your old rag around the tip of the snake. Make it much like a ball. Duct-tape the bottom of the rag to secure it onto the snake. Tear off the tape when finished.
Thread the snake into the duct work. Just as you would in a regular pipe, thread the snake up into the duct, turning the handle and feeding the snake out the whole time. The rag will collect dirt or debris, and you can literally feed out this cleaning snake as it winds its way through all of your heating duct work.
Retract the plumber's snake. When you reach the end of the line, merely spool the snake back into the container, just as you would if this were done for plumbing. The rag will be dirty, so you can either un-tape it and throw it away, or wash it for use later on.
Attach the duct back to the furnace using the clamps and your flat-head screwdriver.