Intermittent flush issues

[MontanaHeaven]

We've had our 310RL since 4/21. NW Montana. Equipped RV concrete pad water, power, septic, skirted. Really no problems at all until things got really cold. We've always paid attention to the weekly dumps, fills, rinses, etc. A couple weeks ago we noticed the "flush " was behaving differently. Upon flush there's like a smallish reverse puff of air coming back out the drain flap while the contents flush down the pipe. Within a day of seeing this happening each time it flushed we could see that the black contents were rising with each flush UP the drain pipe nearer and nearer to the toilet flap. The black tank was only reading ⅓ full.
I have one of those Camco flexible flush wands a friend recommended so I ran that down the pipe into the tank and the black contents seemed to suddenly clear and dump into the tank. Septic dump, big rinse, dump. Black tank read Empty. Seemed to be OK for about a day then again the flushes started doing the exact same thing and I had to repeat this whole process. Couple days of seemingly "normal" flushes then today a few normal flushes then it started doing the same thing with the Puff of air back out the pipe as it flushed. Happened 3 times, however it returned to normal flushing after that.
Sounds crazy and I probably could describe it better. I thought maybe ice, snow maybe a vent pipe.is blocked. The black vent on the roof seems clear enough to act as a vent. The couple times I've used the wand I've run it up and down the pipe and into the tank really giving it all a good spray.

Thoughts?

 

[CornCrib]

I agree with your opinion that its the vent. If the vent were operating correctly, there would be no "puff of air" ... suggesting a build up of unescaped gas.

 

[MontanaHeaven]

I suppose I could get the vent pipe snaked just to see. The Black Rinse components thread mentions an AVB under the bathroom sink. I wonder if that would/could have anything to do with this?

 

[CornCrib]

I"m guessing that vent under the bathroom sink is for the grey tank that the sink flows into ... I don't think it would be a good design to vent black tank into the living space.

 

[CornCrib]

I stand partially corrected .... there is a component for the black tank under the bathroom sink:

"Top Right item: Atmospheric Vacuum Breaker (AVB) or Back Flow Preventer: located (typically) in bathroom in base vanity cabinet or in/near laundry plumbing area (usually high mounted)"

I still think the answer to your troubles will eventually be found in the black tank vent stack.