Well that stinks!

[Jolvi]

So now that campgrounds have opened and we get to take our Jay Feather out and use it more than 4 times I mean with winter and all the hoopla it has not been easy. Anyway we are at a campground and so when we flush the toilet it reeks in here I mean really reeks. Seeing as how it is a full hookup campground I think okay we will flush the black tank as like I said we had only used it 4 times and it was winterized and has sat until this week. Well when we flush the toilet the smell is bad so we hook up everything and let it drain and not our first RV so had a ton of water in it. All comes out as it should, run the tank flush for about 20 minutes while I finish up some other chores. Recharge toilet with chemicals and low and behold it still smells terrible, very determined I am going to figure this out. I know enough and I know they say to keep drain open when using tank flush well I close it for five minutes my wife doing what I asked her to to and flush the toilet and see if there is any odor. Well glad it had been flushed out quite a bit as it spewed out of toilet and onto floor just clear water nothing else. That is why I carry a wet/dry vac for such occasions. My thinking is there should not be pressure on the toilet it should be vented to the atmoshpere from the tank and it should not have pressure on it. Well I have heard about the vent stack dropping into tank so I decide I will get on roof and check to see if that is what it has done. Well to my surprise the cap over the vent stack had popped off I mean it broke the screw down points. I think okay maybe that was already loose so I will just put it back on. Uh no can do the stack is higher than the cap so my thinking is that the cap had basically sealed off any air flow. Seeing as how there are no towns close will fix when I get home, but the good news is now when we flush no odor at all. I know on the previous trailer we changed to the Siphon 360 Gen II and never had odor so guess that will be my next mod.

[Adios Pantelones]

So I'm not sure I grasp your entire post but here are my thoughts:
1. Keep BW valve closed until ready to dump. Not open all the time.
2. Use tank treatment
3. Try to never flush when BW valve is open. Dump tank. Close valve, Add treatment with a few gallons of water and leave it.

Different sites seem to have sewer lines plumbed differently. I get smell at some dumps and not others. With the treatment and water it goes away pretty quickly

[Jolvi]

I appreciate your reply, not first RV and yes we did precharge blackwater tank, and keep blackwater valve closed until ready to dump. The trailer has been used 4 nights and then winterized first time out this year. Odor coming from toilet was bad so we decided to dump it and flush tank which we did, but still had an odor. Climbed on top to check venting and the cap on the bathroom stack was a bit bent like it had been forced down and some of the legs on it even had the screws pulled thru the plastic, go figure. When I measured the stack was higher than the cap so from what I gather the cap was basically sealing the stack so no venting. Will repair when I get home and hope that is the issue and nothing else. One of the advantages of my house is the sewer clean out is close enough to use it but we are at a full hook up site so will drain, flush and recharge system here/

[Adios Pantelones]

I would try and put some washers under the cap legs and raise it up as a temp fix. I'd be afraid with no cap driving down the highway the wind may force air into the tank and back inside the rv.

[Jolvi]

There is still one screw holding it on and a bit of Dicor over existing screws than leg has to last 130 miles till home then replace caps already shipped changing to Siphon 360 GEN II have had good luck with them. Thanks for the suggestion funny part is that I happen to carry washers around all the time to fix the fold up chairs.