I used to have the same problem where my vent lines would create a siphon because they were lower than the bottom of the sealed belly. We tried a couple of things with various success.
DISCLAIMER: The kind folks at Giant RV seem to have been dropped on their heads multiple times, so please don't judge me for their solutions with this warranty issue.
Solution Number 1: "Yeah, that happens sometimes with these units. To break the flow of the siphon, just hold the end of your compressor hose against the vent line and give it a couple of short blasts to clear the line."
That worked just OK, but I knew it was a horrible idea to blast air into the tanks and risk bursting them.
Solution Number 2: "We installed shutoff valves on the vent lines for both of your tanks. Make sure you open the valves while you're filling and then shut the valves when you're done. That way the water can't drain out anymore."
This was a pretty bad idea too, plus it was a pain in the butt to crawl down there to mess with the valves before/after filling. I also knew that this really didn't address the poor design of the vents.
Solution Number 3: I quit dealing with Giant RV and hired a mobile tech to install new vent lines that didn't curve or loop and vented the tanks up high instead of down low, the way a vent is supposed to work.
This worked.
I wish I had bailed on Giant RV a lot sooner.
