Sewer Gas Smell

[elliskrott]

When black water tank gets close to full I get smell of sewer gas in my unit only seems to happen if black tank is close to full. New ball seal in Dometic 301 toilet and it's holding water so don't think it's coming from there.
Any idea what's causing this?

[Brantlaker]

It could be a blocked vent. Pull the cap and run water down it

[JRTJH]

If it "only happens when the black tank is nearly full," I'd suspect that the vent stack is pushed a little too far into the black tank and when the liquid rises high enough, it blocks the escape of sewer gasses through the vent. You MIGHT (some can/some can't) be able to pull the vent stack up a half inch or so. If you go on the roof and try that, be EXTREMELY careful that you don't pull the vent stack out of the black tank or you'll have a mess for sure !!!!! If the vent stack seems like it won't pull up easily, then it's either screwed into a threaded connector that's glued to the tank or it's glued to the tank. If either is the case, then you're not going to be able to solve the problem.

Realistically, if it's only happening when the tank is "nearly full" then the easiest and most sure way of resolving the problem is to dump the tank when it's about 3/4 full. That'll keep the offending gasses vented through the roof stack and keep your bathroom free of the odors you notice when the tank is full.