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Thanks ChuckS, that helped a lot.
I was totally wrong about a concentric vent. I was looking at a pic and the extension on the exhaust tube is to allow for the distance from the water heater cabinet and exterior wall on the camper. Disregard my previous comments.
It's a little confusing because it's shown on one drawing in the manual, but not on another one, but it shows the left side access panel in figure 9 having 20 perforations of unknown size. Note, they call it the left side as viewed from the rear of the water heater where the water and gas connections are. I'm going on the assumption this is where combustion air is meant to be pulled in.
Is it possible to pull the exterior cover off and look in there with a flashlight or a borescope? Since you say this problem has been prevalent since you bought the Montana, I'm now suspecting maybe it's installed a little close to the bulkhead or maybe a piece of insulation or some other material has fallen against the panel.
If you don't find an obstruction, maybe your idea of punching a vent hole in the outer cover might be the best fix. It's hard to believe, but if the outer cover is caulked in, the compartment is just too air tight. I doubt you need a 3" vent, a 2" should be adequate. If I'm looking at the drawing correctly the perforated panel should be on the right hand side looking from the camper exterior. I would punch the hole lower than the exhaust and favor the right side as much as possible. You don't want to set up a scenario where the intake is trying to pull in the exhaust gases. Try to find some kind of screen to cover it, maybe order a replacement for the exhaust and use that, it would look better.
Are you getting any error/fault codes on the controller when this happens? The table shows an E6 error for air pressure fault, and an E2 error for accidental stall which is without flame during combustion. I would think one of these would come on.
The gas you're smelling before ignition and after burner cycles off might be normal. This unit has a 20 second post purge after the burner goes off. The draft induction blower runs over to purge unburnt gas and will also help purge any carbon monoxide, it's a really good safety feature. When the combustion cycle starts, the draft induction fan is the first thing to start and should there be any remaining unburnt gas in the burner, it would purge before the burner fires.
I was looking at the specs on the Suburban model and was surprised to see it's only 1.5 GPM. The Fogatti rates nearly twice that with a slightly smaller burner. You're probably right in that it was installed because of the tub, but the shower must be very weak.
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