It is a common problem--and IMO the worst idea SeaLand ever came up with because it doesn't allow a tank to "breathe," which is the key to prevention holding tank odor
(odor out the vent).
However, you're saying that the whole cabin
stinks, so it may not be the source of your odor. A little investigation is needed before trying to "fix" it. Odors are always strongest at their source. So next time you come aboard, first go straight to the tank and stick your nose there. If it's really strong there, then the pressure relief valve IS the source....but it may not be the valve itself. You self-installed it in a PE tank, so you may not have sealed it correctly. If that's not where it seems to be strongest, it could be residual odor left from the hoses you replaced....or it may not have anything to do with your sanitation system. A wet dirty sump or bilge
can make a whole boat smell like a swamp or even a sewer. Or there could be trapped water
somewhere that's stagnated...that can also make a whole boat smell like a sewer.
Once you've located the source, eliminating the odor is actually pretty easy, though it may require a bit of manual labor. Let me know what you find.
"If you can't explain it to a six year old, you don't completely understand it yourself."