Annoying water leak/drip from drain tube

We have an older early 90's Jayco Eagle that has been a great trailer but I have had an annoying leak/drip coming from what I guess would be a drain tube from the back of the trailer (next to the sewage drain) when the water system is pressurized. We have had the trailer for over 5 years and it has always had this leak/drip but it has really gotten bad and wastes a lot of our water we take. I cannot trace it back as it is sealed where it exits from under the trailer. If the water system is pressurized it really leaks/drips good and if you open up the water faucets and un-pressurize the water system the drip from the tube pretty much stops.

I have asked numerous RV repair places about this and nobody seems to have any idea what could be causing this. Any ideas would be greatly appreciated.

thanks
Scott