I have a 20lb bottle with that gauge - we always purge the lines of gas when we finish BBQ'ing and the gauge never drops. Only time it reads in the red is when the tank is empty.
If that gauge is after the shut off valve and it drops - where is the gas going? While I understand the reason behind a self purging system, in the wrong situation that could be dangerous.
How old is the trailer? Older hoses or pipes could have been cracked or nicked when the new device was installed. I remember fixing air leaks
on old semis that still have copper air lines. When you tighten one leak, a fitting 10 feet away got jarred and started leaking or cracked and had to be replaced. Newer synflex hoses helped but only as much as the quality of the installer.
Sorry to ramble, but I would track down where that gas is going.