The anti-vacuum devices (in-line vent similar to this one:
https://www.hardwareandtools.com/oatey-39012-inline-vent-ecja-8601.html ) above each trap should prevent P-trap emptying, but not if road movements empty them or a device has failed. I suspect that’s what’s wrong with my shower drain issue... a stuck-closed inline vent valve that allows the infrequently used shower trap to vacuum dry each time I empty the gray tank. If there isn’t enough water weight in the trap to be more than the inline vent’s spring strength, the spring may keep the vent sealed, and the rest of the trap water gets sucked out. I don’t know that it’s too hard to check and remedy the spring/seal movement.
Not sure how you “upgraded” the under sink vents or why you’d have additionally “plugged” them, unless it was an Oatey Surevent, a Hepvo, or something like this
https://www.ipscorp.com/pdf/studor/STU021_TrapVent_web.pdf . It may have been unnecessary if the Splendide was the culprit all along.
You’d think a roof vent would prevent siphoning, but a one-way valve there may mitigate air coming in via the roof vent. And as to your combined vent question, I’ve heard of that; but the ‘06 Monty’s I’ve seen all had two side-by-side “T” roof vents, as does mine - so reckon I can’t offer much insight there.
Also it’s advisable to not leave the dump valves open to the park sewer while camped. Accumulated tank fluids flush solids best all at once.
Joel