Foul odor in bathroom when vent fan running

[EngiDent]

Hi
2000 Land Yacht 355XL.. Have had the motorhome for a year and a half now. When we got it, the bathroom vent fan (Fan-tastic Vent) was not working.. Loose/corroded ring terminal at the plunger switch. I fixed that. Now, when the vent is up and fan is running, the bathroom fills with an indescribably foul odor. It is not at all like septic gas. If you've ever had to change an adult diaper on an elderly person who has not been bathed in several days, it is more like that. Definitely a biological odor but I do not believe it is coming from the septic tank. If I hold the toilet flap open, what little odor I get from there smells like a regular septic smell (the worst of which I've ever smelled is more tolerable than what I'm talking about) dampened by the blue stuff I put in there when we parked. What am I missing? Turn the fan off, close vent, the smell is gone within a couple minutes and I never smell it under any other circumstances. Most bizarre thing I've ever experienced in 35 years of RVing. Thanks!
Dave

[Minno]

Do you have a grey water tank? Grey water can smell extremely awful at times. I'm wondering if you have a blocked grey tank vent, or the fan is pulling the odor up from the grey tank. Only thing I can think of if it's not the black tank. Might be worth flush your tank out well.
Good luck!


Kay

[EngiDent]

I cannot see evidence of a vent for either tank on the roof, which is another mystery. I'll try flushing the grey. Thanks!

[ITSNO60]

Also check to see if you have one of these defective Dometic 300 toilets

https://www.youtube.com/watch?v=mNqwfe2gtS4

[richard5933]

Lots of places around a toilet for urine and other odorific material to gather. It's likely been there for years. Also, it's possible that there is material inside your toilet between the different layers of the shell.

Might be worthwhile to remove the toilet and clean it inside and out, including between the layers of the casing. Good time to replace the flange seal just in case.

Do you have an unused gray water drain? Like a shower you don't use? If so, that p-trap is likely dried out and not providing an odor barrier.

[EngiDent]

I had to pull the toilet last year to replace the water valve. Toilet was nasty. I hosed it down inside, out, and underside. Replaced flange seal. Cleaned all around the toilet, wall and floor, with oxy cleaner. But the rig definitely belonged to an old man with bad aim. Question.. Does the grey tank overflow into the black tank? I tried filling it up to flush it, but after the meter said full it still didn't back up into the shower or on the ground. Meter says black tank now 2/3 full but I don't trust it, something tells me it always says that. But it's only been 2 days since I dumped and rinsed the black tank.

[Wazbro]

Some sinks have air admittance valves that sometimes malfunction and need replacement.

[jrwayne]

on occasions we would get an odor coming from the shower drain in the gray tank. I put in a capful of tank techs Rx and it took care of the problem. We use tank techs Rx in both tanks regularly and everything is always as it should be.

check out their website:

https://tanktechsrx.com/

and I am not affiliated with the company. I just really like their green product.

[Thonevan]

Make sure there is water on your toilet bowl. Also water in your sink and shower trap

[NOAZRK2690]

First eliminate the black & gray tanks. Empty both tanks then dump something like Lemon Pine-Sol in the tanks and fill both to the very top. I use like a half gallon in each tank. Let them stand over night then dump both tanks. I'd also change the toilet wax seal or at least make sure the bolts are firmly tight. Then see how or what the fan draws. If your battery box is close by the bathroom that would be my next target.

[Foiled Again]

I know this is a moho, not a trailer, but at least one trailer owner found that the vent pipe for his tank had never been installed... yeah! If you over fill a tank with the tank washers, it is supposed to cascade down the roof. His just burped on the trailer floor.

[EngiDent]

Update: problem was that the toilet flange seal was folded over when the toilet was reinstalled, and one of the bolts was missing. I had pulled the toilet last year, but a friend reinstalled it. New seal / bolt kit and the odor is gone.

[richard5933]

Glad it was something so simple. Thanks for posting the followup. Love a good mystery - but it is nice to finally find out what was going on.