Put about 10 gallons of water in the tank and go for a nice drive with the camper. Before you do, put a cup of Tide or a few good squirts of Dawn dishing liquid in there too (or both if you want to be real adventurous!) Yea, it will bubble up, but it will go away too, leaving your tank smell much better when you flush it the next time.
Another help is to put about 10 gallons of water in a couple cups of bleach. Go drive around, but drain the tank and flush it out immediately so the bleach will not deteriorate your slide valves. But this, definitely cleans up the smell.