Odor coming from behind washer

I don't know what the smell is but Im pretty sure others have already posted, but I had a similar situation because I think my drain pipe is straight down into the grey tank and a bad oder was coming from the W/D compartment. I just took some silicone and sealed it up, No more smell and I also installed solar tank vents.

X3 I run mine on a rinse cycle when I winterize with antifreeze and again with fresh water when i dewinterize.

as OLD-Biscuit replied you p-trap has lost its seal as without use the water will evaporate. Another solution if your not going to seal the drain would be to pour some mineral oil in the drain the standing mineral oil little to no evaporation rate will keep the gas from venting back into your rv.

That washer drain pipe typically goes to grey waste tank (some go to black waste tank)
There SHOULD be a 'P' trap in that drain line to prevent odors coming back up thru line from waste tank...........probably doesn't have any water in that 'P' trap anymore.
If you aren't using it the water evaporates.
If you are going to use it........put a cap on that line and seal it.