The 'air admittance valves' under kitchen and/or bathroom sink may not be staying fully closed and the furnace return air draw could be sucking odor up thru drain line from grey tanks....they can smell just as bad as black tank.
Sewer smells inside mean something is wrong
Toilet bowl not holding water (water is vapor barrier)
Toilet floor flange not sealing (mechanical barrier)
'P' traps going dry (water is vapor barrier)
Air admittance valves not closing (mechanical barrier)
Vent/drain lines from/to waste tanks cracked
I'd suspect the air admittance valve(s)
Inside of rig is cold..furnace is turned on......return air flow pulling odor up from grey tank thru AAV inside rv.
They are on sin drain lines after 'P' traps on a short stub pipe. Just hand tight. Available at most any hardware store for a few bucks
