Source of pressure doesn't matter as long as there is some pressure. Agree that it must be a bypass valve somewhere. I had the same problem first time out with my Dad's RV. Took me a couple days to find it under the bathroom sink behind a small panel that was screwed in. Was getting a little ripe by then.
Most newer rigs have it in the wet bay.
Maybe post your year and model if you are still having trouble finding it.
97 Fleetwood Bounder 36S on F53 chassis 460 with as many quads on the trailer that I can fit or get running!