I've got a new 2013 305V12, all of 6 weeks now (been out 4 of those weekends) and I'm starting to have a few problems with the unit. If I fill the fresh water tanks near max then it starts draining out the overflow. Problem is that it doesn't stop draining until I am down to 1/3 fresh water. Is there a vacumn issue or away to resolve this?
This problem can be caused by a poorly routed and/or overly long vent hose.
Not an easy fix, so the "plug or clamp the vent hose" trick might work for you.
If you want to fix this permanently, Here are some suggestions and photos of the area (not all of my camper).
My vent hose goes pretty much straight up from my tank; Through the compartment behind my WFCO power center; and up to the fill port right under the fridge inlet vent.
Even though pretty straight, it still manges to push water out the vent long before the tank is full because when connected to the TV (like at a water fill station) the tank is rotated down towards the fill, vent, and water pump hoses.
When hooked up, about 3/4 tank, the vent hose is under water. This pushes water out the vent and, because the hose pressure can put water in faster than the vent can relieve it, the tank bulges and burps the water out the fill port when you remove the hose.
The tank must be exactly level (or in my case even tilted slightly to the rear of the camper) to get a 100% fill.
Wind blowing past the open screened end of the tube, causes a low pressure to develop at the vent hose inlet. The Venturi Effect acts on the hose end sucking the water out. A loop in the hose makes the pressure required to start the siphon lower (the water is closer to the open end and easier to get the siphon going) and campers with a looped vent hose see this effect a lot more often than those with a straight hose.
Driving down the road; normal sloshing of the tank and the Venturi Effect of the wind, will pull and slosh water out the vent line starting the siphon.
1) Drop the belly panel and make sure the hose has no low spot where water can pool so air can escape the tank. (As straight UP as humanly possible.)
2) Install a door (locking) on your fill port. This serves as dual function of limiting the Venturi effect and securing your fill port to casual vandals.
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I've had simlar issues with my 2501ss. Would be driving down the road and see water dripping out from the overflow. I just put a small piece of duct tape over the overflow vent/screen by the water fill and don't have the issue. I remove it once I'm at the campground to allow the tank to breathe when pumping. The tape is easy to take off and replace each time.
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