RV guy was just here. Can't see any obvious leaks but says the rubber is checking. Thinks it's getting porous and should be replaced. He thinks scrubbing and prepping to apply a coating will make it worse, and isn't sure it will stick (like duct tape on a powdery surface is how he put it). I did try some Eternabond at one point for an emergency patch and that did not stick despite cleaning and rollering it hard, so maybe he has a point there.
RV Daytrader: yes, installed roof vent/fan.