DO you leave your gray valve open? DON'T. For now,, I'd you might try this.
Plug drain. Dump tank (Gray) Disconnect hose.. Stuff a can of fogger up the pipe and put the crain cap on. Come back in 2 hours and remove the can, close the valve (in the opposite order) that should gas 'em.
NOTE> might be a good idea to be elsewhere for those two hours.
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times