[JRTJH]

I'd suspect that if it is a black tank leak, once the odor "increases after the 4th or 5th day, it wouldn't "disappear after sitting a week with no trailer use"... Once that smell dries on the top of the tank or on the coroplast under the tank, it's going to be there every time you open the passthrough or every time you flush the toilet... Odors from the black tank don't "come and go" like that.

I'd guess that it's air infiltrating the black tank contents "somehow" and migrating up into the trailer cabin "somehow"...

If it was "liquid escaping the tank" the odor would linger much MUCH longer than a day or two.....

The toilet, if it's a Thetford, is a likely source, as is the foam rubber seal that is used in trailers in place of the wax seal that's used in S&B houses.