I watched a couple of videos on installing/repairing this water heater including the one with the spiders nest. After watching, I retract my previous retraction from post #11; it definitely is a concentric vent. the vent insert that goes through the outer door has an inner ring that I'm guessing is 2" diameter and an outer ring that appears to be 3" diameter. The red rubber gasket on the water heater seals against the outer ring, the inner ring slides into the exhaust tube on the burner housing. It's supposed to draw fresh air around the outer circumference of the vent screen in the outer door. It's not the best design I've ever seen, but they didn't consult me.
I seriously doubt the ignitor is an issue. It only operates until the burner fires and flame sensor detects flame, and this is not the reported issue. If the unit does fire properly and then drops out, it either loses air, fuel, water flow, or a bad flame detector.
I did notice in one of the videos that Suburban uses a diaphragm type single tube pressure switch to detect combustion air flow. These things are notorious for filling with condensate and failing. Looking from outside the camper, it' on the left hand side, need to remove that panel. It's on the negative pressure side of the fan and should be easy to test. Unplug the tube at the fan housing, put an ohm meter across the switch terminals, suck on the tube and see if the contacts close; also check for condensate in the tube too. You might possibly hear the contacts click if you don't have a meter. I would only jumper the switch for testing purposes only, do NOT operate long term with a jumper, very dangerous. According to the online manual this is part #233229 and I saw an online vendor had one for <$30
https://rvpartsexpress.com/product/s...switch-233229/
Although a possibility, I'm still not convinced it's the root cause. If it works with the cover off, the switch still has to work, it's just getting it's air from another direction. The most likely issue is blockage in the intake, screen, or venturi. It's highly unlikely to be an issue now, but those vent screens can freeze over in below freezing weather. The exhaust is very moisture laden and if the screen is very cold, condensate can start falling out and collecting on the screen. Once the water heater cycles off it can freeze over enough to block air flow.