First of all I'm new to this site. I traded off a 2015 Montana 3720RL for a 2018 Redwood 3901WB and I have a question. Where is the back flow/vacuum breaker for the black tank flush located? I would like to omit it or replace it with a real one. I've replaced the West Lake tires with Goodyear G114's and I have an appointment with Moryde to install their independent suspension/ disc brakes with Timkin bearings. I'm installing a sleep number bed and working on getting 110v line to the bed slide for the pump. I have to insulate the front cap, safe area and add insulation to the underbelly and I should be ready for a shake down cruise to find the real problems. Any other suggestions would be appreciated.

Welcome aboard!

On this website, you'll find PLENTY of suggestions to make your unit more efficient and your own.

As always, if you have specific issues you need help with, there are a lot of great folks here willing to lend their knowledge and experience.

Double check your A/C registers. They make be long and prevent air flow. If you don't have disc brakes, you won't go wrong with that modification.

So many things to do and see...

Again, welcome to the group. :cheer:

We have a 36FL and ours is located under the bathroom sink. Unless somebody else knows for sure, that's where I would start looking.

Same on our RL coach

I'm not positive, but I think it's in the wall at the end of the shower and washer hookup. If you don't have a washer pull the access panel and check. I would on ours, but we have the washer and dryer installed.

"Where is the back flow/vacuum breaker for the black tank flush located?"

You are asking about the black water flush connection with a backflow/check valve and not an anti-siphon fitting, correct?

In the Utility Bay, where the water hose connects to flush the black tank...

Go into the basement and remove the side panel. You'll need to access the back side of the Utility Bay to remove the hose fitting connected. Then remove the 3 screws in the front. The check valve is located on the backside of the water hose fitting. $18 at Walmart.com. Less at Amazon or Ebay...

You're not talking about the Indoor Vent Stacks are you? There are at least 2 possible locations (depending on model). Maybe more.

One is located under the kitchen sink or adjacent cabinet.

The other is under the bathroom sink.

If you have two bathrooms, there could be a third one.

There could also be one behind the Washer/Dryer.

Home Depot - $20 each.

Then, to make sure again...You aren't talking about the HEPvO anti-siphon Plumbing traps are you?

There are at least 2. One under the kitchen sink or adjacent cabinet and one under the shower...$33 at Amazon.com...

He's talking about the backflow/anti siphon on the black tank flush line.
All the above is good info, but wrong items.
As others have said, mine is under the bathroom sink, up high so it's actually behind the sink, all I can see is the 2 lines going up to it without pulling them down a bit.

Thank you all for your help. The Anti siphon/check valve for the black tank flush line is located behind the shower wall and is accessible from the closet with the washer and dryer. My SOB had a cheap plastic model that eventually leaked and I replaced it with a brass one. I want do it now as part of preventive measure.

A photo of this valve would be greatly appreciated if anyone has one. I'm stumped as why this would be in the interior of the coach and not in the utility bay (furnace/water heater) right behind the black tank flusher connection as GipC indicated?? If anyone know the location of this in the GK that would be appreciated as well.

thanks everyone!

Maybe this is what you are looking for?

The anti-siphon valve has to be higher than the tank. Based on where the Black tank sits and where the valve can be placed high enough, that's where it will go. In the case of the 38GK, its in the shower faucet access cover in your master bedroom. You will see a wood plaque on the wall that does not look like it belongs there...

These are cheap plastic ones. They make brass ones if you prefer.

B&B Model 571
Amazon: $30

But if you want to save money and get a better fitting (brass), note that the one just below it is $19. You'll have to buy a couple of brass 1/2" Nipples but why would you want plastic for this price?

Maybe this is what you are looking for?

The anti-siphon valve has to be higher than the tank. Based on where the Black tank sits and where the valve can be placed high enough, that's where it will go. In the case of the 38GK, its in the shower faucet access cover in your master bedroom. You will see a wood plaque on the wall that does not look like it belongs there...

These are cheap plastic ones. They make brass ones if you prefer.

B&B Model 571
Amazon: $30

But if you want to save money and get a better fitting (brass), note that the one just below it is $19. You'll have to buy a couple of brass 1/2" Nipples but why would you want plastic for this price?

Thank you GipC!! I'm familiar with this panel in my bedroom and assumed it was only for access to the shower valve. I love these forums!