Here is my experience from last summer/fall. We purchased a used Cougar 29BHS and it wasn't winterized properly as both the outside & tub faucets, and the plastic anti-syphon valve were cracked.
I replaced all of them, but figured I would put in a brass anti-syphon valve. Fast forward a month or so and I noticed the wood under the bathroom vanity was expanding from moisture (I would love to find a piece to replace it). I checked under the sink no sign of leaks, I checked under tub and figured it was running under there during use and coming out during travel, nope not the case also checked the roof, really was scratching my head.
brass valve:
https://amazon.com/dp/B00467HJBQ/
What the problem ended up being was the brass anti-syphon valve. It requires a minimum pressure to run or else it leaks, as when I would start and stop and it would leak both times and also during if pressure was low.
I fixed it (probably not the right way) by putting a elbow in with a 1 way valve too (took the 1 way valve off the anti-syphon and the brass union)
Seemed to solve my problem and I also use another anti-backflow out by the hose connection so then it would take 2 valves to fail.
I did this on our last trip last year and haven't been able to confirm if there was any adverse effects to flushing, syphoning, etc.
At the time I had found some posts on either here or another forum of other people removing there's after having issues.