Your profile says you have 3 trailers, so the solution may depend on which one has the problem. One member with an almost new trailer removed the wall panel to inspect his vent pipe and... it was missing! Smells sort of wafted upward but every flush injected nastiness into the trailer. Seems the vent pipe may possibly simply rest over a hole in the top of the black tank and sometimes the pipe falls into the bottom of the black tank where it can't vent at all. If the vent pipe is in place and intact, it can still be clogged by mud dauber nests, etc. Before major surgery try running a small sewer snake down the pipe for a simple free fix.
Cleaning the tank with a toilet wand inserted through the flush valve is more effective than any flush system Airstream ever installed. Old ones were made of copper, newer ones are plastic, and the cap on the end can fly off while in the first use. Make your own or use PVC pipe glue to secure the end cap before use.
Check the condition of the floor surrounding the toilet, too. Calcium buildup on the black rubber seal can cause water to slowly leak leaving a dry bowl. Using "Lime Away" 3 times just let me delay the (gag, retch) job of replacing the black seal. I know I may have only bought a short reprieve but Whew, worth it. When I do have to put in a new ring I will rinse and refill the tank 3 times, using bleach, Calgon and Dawn detergent first.
Last, the flapper itself can be too worn to hold water even with a new seal, so consider getting a new toilet too.
Today is a gift, that's why they call it the present.