To answer your question---NOT common to have the vent pipes leak. VERY RARE. Now, what you need to do is determine which tank is causing the odor. 99% of the time it is the GRAY tank, not the Black. Dump both tanks and fill and flush them both. Put a qt of Pine Sol in the gray tank with about 10 gallons of water and then drive the unit. See if you get the Pine sol smell. If so, you have a vent issue with the gray tank. If no Pine sol smell, flush the gray and do the same test with the black tank.
Common things that cause a tank odor
1. washer dryer prepped or w/d installed---the p trap has no water due to non use and the odor comes up there
2. Having a roof vent OPEN and the roof top tank vent, the odor is pulled in in transit thru the open vent
3. NO water in one of the sink/shower p traps(but if you use these sinks/ shower they will have water)
4. IF the 1.5 inch ABS vent pipes are not sealed in the top of the waste tanks or loose, you will get odor. The easiest test for this is to fill both tanks to where water comes up in the toilet and water flows into the shower pan. Once that water is there you check for leaks flowing off the top of the tanks.
5. LONG SHOT, but I have seen it. The wash bay, the customer keeps the dump hose and such and does not clean it out and the water bay smells horrible. THAT odor WILL migrate up into the RV as no RV has the water bay air sealed from the inside. Doug
