Odor problem - any ideas?

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We are having an odor problem, but it only shows up when we are moving.  The odor is stronger in the bathroom, but it is also in the cabinet under the kitchen sink.  When we are parked, even before emptying the black and gray tanks, it is fine.  With the black tank about half full the place reeks when we get on the highway.  We parked for several days and emptied the tanks before driving again.  We did notice some odor, but not nearly as bad as when the tanks needed to be emptied.  We have driven with full tanks before and didn't have this problem.

TIA!

 

[Terry A. Brewer]

Odors while moving are almost always caused by losing the seal from a "P" trap. The shower is especially suspect as it has the shallowest one of all.  Grey tank odors smell as bad if not worse than the black tank ones.

 

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Thanks, I'll get dh to check it out.  ;D