fresh water tank vent

My '95 Thor 5th wheel fresh water tank does not seem have to been equipted with a vent. I notice this each time i try to fill it. Cant seen to get the water to flow into the tank it seems to be air bound. I am woundering is it possible the factory just did not install one or could it be pinched or cloged? I am considering punching a hole in the top of the tank and running a tube for a vent but am concerned about how to seal around the hole so as not to allow leakage. any advice?

C Nash

Senior Member
fresh water tank vent

My guess is that there is a vent and it,s kinked or cloged like you suspect. Are you filling to fast or to much water flow into the tank? Punching a hole would be a last resort for me. Have you tried to contact Thor?
fresh water tank vent

I can seem to find any vent at all. although it is not easy to inspect the tank because of the location. not much room. I would expect the vent to be atop of the tank.
fresh water tank vent

The vent on my MH tank comes out right at the fill point. However, I still can't 'force' water into the filler pipe. The vent can't handle a really fast fill.

I've been told that some fillers have the vent hose running right up through the filler hose. They 'glue' the other end onto the top/inside of the tank. The air comes out 'around' the fill end.


Senior Member
fresh water tank vent

I would be surprised if there isn't a vent of some kind, even though it my be poorly installed. If you can find a way to install one, you should be able to locate an existing one. There are several things that are well known to cause problems with fresh water tank vent lines. The plugging with some type of debris is only one, but is worth checking.

You are correct that a vent line would have to come either from the top, or near the top of the tank. They are usually made of clear plastic tubing, but other things are at times used. If you can locate any plumbing line that enters at the top of the tank, it is safe to say that it is either the fill or the vent. I would start by tracing the fill line back to the tank and watch for a location where you have both in close contact.

Another issue in the search is the type of fill that your RV has. If you have one of the old style fills that has a place to insert a hose, the vent line is usually connceted to a small hole at the top of the fill access. But if you have a tank fill that just uses a valve from your city water connection, that will probably enter the tank near the outlet for your water pump.

A properly desinged vent line will exit the tank at, or near the top, and it will travel up for at least a foot or two. From there it can either continue up to a high point exit as with the old style fill, or it can reverse direction to exit from the bottom of the RV. At no point should there be any "U" shape in the line or it will fill with water and no longer vent the air from the tank. That same problem is common in vent lines from fuel tanks on motorhomes.

I would be very slow to "punch a hole in the top of the tank" because you must know how to seal that hole or it will always leak. Most of the modern water tanks have the penetrations "spin welded" into them. What that means is that the fitting is spun to cause friction to melt the tank wall as it is inserted. The wall and the fitting then bond much as a welded fitting would in metal. You will have a very difficult time installing a new fitting in any other manner with most tanks.