I've been asked to comment on this thread, so here goes
Two of our Customer Service people and our Manufacturing Manager just read thru all of these posts together. It seems like there are several different issues being discussed that are actually not necesarily related. Here's sort of a recap:
Post 1 - Water "gushes out" when you open the cap and the tank is empty. Do you mean when you're trying to fill it gushes out? Water gushing out from an empty tank doesn't make sense other than maybe a cup or so that might be caught up in a low point in your fill hose. That doesn't seem likely since it would not be under pressure. However, water gushing out definitely can happen when you are trying to fill.
Post 2 - Confirms water gushes out when overfilling - agreed. Implies that a low point in the vent line is filled with water making it more difficult to fill the tank and more likely that water comes back out thru fill - agreed. However, does not answer question why water "gushes out" from an EMPTY tank when taking cap off.
Post 3 - Did you open the low point drain for the tank or the water lines? Opening the water line low point drain would have no effect. Water tank low point drain would solve pressure situation but water would start coming out the low point drain very quickly. If water isn't coming out quickly either you're opening the wrong drain or there might be a stoppage on the tank low point.
Post 4 - There are 3 low points. 2 for the water lines and 1 for the tank.
Post 5 - Prime Power is a nice guy saying "thanks" twice
Post 6 - Dealer comment is accurate. We have worked hard based on dealer feedback to eliminate low spots in both our fill hoses and vent tubes. Vent tube is industry standard for the most part. Doubt the diameter is the issue.
Conclusion - it's difficult to piece all of this together based on the available information. However, if you have a low point in your vent tube that is now full of water from a previous over-fill it will act like a P-Trap and make it very hard to fill your tank without the water backfilling thru the supply hose. M109s advice to "blow out" the tube is good and will help. Even for a non-clogged vent tube it is not unusual for the flow of water going in to "over power" the vent tube and cause water backfill in the supply hose. In that case, turn down your water pressure when filling. If water is coming out from an EMPTY tank, we're up against something pretty unusual.
I suggest contacting our Customer Service group so we can have conversations regarding specifics to help you out. Try them at:
Prime Time Manufacturing, Manufacturers of travel trailers and fifth wheels
Sorry for the long answer and the lack of true answers, but I hope this helps to some degree.