Check the vent line for blockages or snags. Make sure the vent line is short and horizontal. A standard size vent line is 5/8th's of an inch. For larger tanks, use a 1 inch line, and if the line has to run around a 45 degree corner, you can use a larger line, but no larger than 2 inches.
Backflush the vent line to remove blockages if necessary.
Rinse the holding tank with one part bleach to one part water.
Use a bacteria eating chemical that is environmentally safe to sit in the holding tank once it is thoroughly cleaned of old waste.
Keep the system emptied when stopping at receptacles. The best way to limit the amount of odors your system puts out is to frequently remove the waste contents.