I know for sure it is filling while underway. I know the bags are bone dry before they fill a little. I know this because it was part of my testing procedure on my new setup.
I did break down and call wakemakers and get the vented loop. I will go half the summer trying to find a cheap way to aleviate this and throwing parts at it or do it right. I called and talked to Jason and he spent a good 40 minutes on the phone with him. I decided to get the 3/4 loop and 3/4 hose and change over from my 1" discarge and 3/4" inlet from the pumps just for flexibility and easier to hide and bend. That was a lot cheaper than doing the 1" vented loops as well and the pumps I am using have enopugh flow for a 3/4 by a long shot (atwood 750/800s). I figured I would just make it easy and do my best to make it look factory. I tested my sprinkler valve in a bucket and it seemed to flow pretty decent, but it required water pressure to reference, which I dont have now because of my cut scoops. If I had not dont that, I would probebly just have used them and called it a day. I know a lot of people are redoing their system that has those, but I figured it does what I wanted and didnt require cable actuated valves. That was all I was really after, but I think I will be happy doing it right this time.