Gray tank capacity for Citation/Siesta/Synergy

[KC8RXQ]

I apologize in advance if this is a re-post but I couldn't find where I logged the original.

My Synergy brochure indicates a gray tank capacity of 24 gallons but there is no way it will hold that amount before the it starts backing up into the shower pan. It is on my list of things for the dealer to check but I was wondering if anyone with the Sprinter Class C's in the rear twin bed configuration (Synergy TT24 or Siesta/Citation ST24) had a similar situation.

I did experiment with the capacity and measured around 12-15 gallons of water into the gray tank before the shower drain showed signs of starting to back up. Yes, the rig was very close to being level on both front-to-back and side-to-side planes but it's a mystery as to why such a difference between the brochure and actual capacity. I have gotten used to draining the gray tank when it registers 2/3rd full on the monitor panel just as a precaution.

Any thoughts or suggestions would be greatly appreciated.

[Travelin' Texans]

Unless yours are better than mine, the sensors are worthless. So if you rely on your sensors to tell if it's empty, it's probably not.

[KC8RXQ]

No, I don't rely on the tank sensors and I learned that valuable lesson on our 5th wheel. I do use it as a gauge since the 2/3rd light seems to coincide with the point at which the shower drain stops draining effectively.

Regards,

[Travelin' Texans]

It almost sounds like they extended the shower drain into the tank too far, so once the level reaches that point it doesn't vent properly & either drains slowly or backs up. I'm positive they did that with the roof vent line on my grey tank, it drains very slow at first then it gurgles & drains quickly.

[Hooligan2]

I have not measured the capacity of the grey tank, it does seem correct. Mine does not backup into the shower even when indicating full. Unlike Travelin Texans it takes several minutes to drain the last 1/3 of the tank to get the empty light on...
The tank is large but very flat, with the drain more at the front than on the bottom???. It would drain easier if RV is slightly nose down and may not drain completely if nose high.

Since the tank is forward of the bath, it may backup to the shower sooner if the RV is slightly nose up. Or if the shower drain pipe goes uphill to the tank. Dealer should be able to check that..