Odor in cabin

This was probably mentioned...
I eliminate 90% of odors by pouring Clorox down the drains, showers primarily. Stop peeing in the shower
second look is the holding tank vent filter. replace annually
Give those old head system hoses a sniff. they turn tan when they are overdue for replacement.

I've had two "odor incidents" since I've owned my boat.

Number one was caused by a clogged vent in the waste tank. Pressure built in the tank until the sewer gas was forced past the joker valve and exited through the water in the toilet. A properly functioning vent allows the system to work properly, during both flushing and pump out, and by introducing oxygen to the tank, it prevents the tank from going septic.

Number two was caused when I inadvertently shut off the shower sump pump, which also pumps the a/c condensate overboard. The condensate drained into the bilge and became funky, to say the least. I remedied that situation by pouring a weak bleach solution in the bilge, letting it sit for a week, then pumping and vacuuming the bilge dry.

My funky bilge problem produced an odor similar to your description. It was somewhere between diesel fuel and used motor oil, though not as strong as diesel fuel.

Another vote for an ozone generator AFTER you fix the source. Never had the need for one in my boat but it's basically standard procedure with neighbor who flips boats, to run one over night in just about every cabin boat he takes in. Only time I used his was after "theft" of a ground floor rental unit that owner was disgusted with due to musty smell. Water proofed all around the exterior ground level wall, 24 hours with the ozone generator, followed by a good airing out with fan only running and the smell never returned from over 6 years ago.

I'll never again gripe about the faded green stripes & dirty old carpet mine came with.

I would highly recommend Noflex for holding tanks. I used it for years and it was so much better than any of the other green or blue chemical products. Odor issues disappeared and sludge buildup was greatly reduced. Very good product.

We put these in our boat, it helped.

NuShield Air Ionization Systems Nu-Calgon (nucalgon.com)