Fresh Water tank ( and indication)

I have just come back from a 5 day camping trip where we not able to connect to city water. After filling the fresh water tank to overflowing, then moving onto a very tight camping spot, I checked the levels in my tanks a saw that the fresh water tank was only showing 1/4 full. I thought I had an indication problem but after one day of camping the water pump was having difficulty keeping pressure in the system.
After a couple of rum and cokes I figured I must have venting problem since the 3370 has a 75 gallon fresh tank. Eventually after taking off the storage area wall to see the lines. I held a tube up against the vent grill cover and blew, I felt back pressure and then it cleared. I carried six more bottles of water (30 gallons) in a baby stroller and managed to pour them in via the tube and a funnel, the indication now went up to 1/2 full.
Therefore if anybody is having level indications issues and cannot store more than 20 gallons or so blow down your vent line you may save yourself a visit to the trailer repair shop. Other than this as it has turned out little problem I am a very happy Bighorn owner.

Chip, what seems to happen is a small amount of water may get trapped in the vent line in a low spot almost like a P trap and cause the fill tube to back up and seem full while filling. What you did afterward by blowing back into the vent tube is the best bet. I have a garden hose adapter that I use for filling and it fits pretty snug in the fill hole. I constantly check to make sure air is coming out the vent line and once I get a lot of water out of the vent, I know it's full. Sadly the level indicators leave a lot to be desired.

Chip, I have a 3370 as well,I had the same problems as you did.Blowing out the vent line does work but the problem keeps coming back.I pulled the basement wall out and cut the plastic underbelly so I could fold it out of the way,I found the vent line taped to the fill line laying at the bottom of the tank.I rerouted the vent so I could fasten it to the floor now it works great.I have to gravity fill the water tank every weekend when I leave the 5er at my camp in the mountains Good Luck Jon

check before you park & setup

Thank you for the advice, next time I fill, I will check the level indication before I leave the filling station. I have since filled and emptied the tank at home as a test with no issues, but I did not fill to overflowing thru the vent. I will try that next time before we go on our next trip in about a month, that will show if I have a P-trap situation or low point. Sadly I have to store my Bighorn at a friends farm. One of the problems of living in a subdivision