Need to be careful since you're dealing with gas and calling Dometic is one of the safer choices.
I'd check inlet for air and make sure it has good flow. If the outlets have good flow, you would think inlet is fine as well.
Are you at elevation? Possibly propane/air mix is off? Usually need to be at least 4,000 feet up to be an issue.
If you let the hot water run on gas for 15 min or so and then try the heater does it work? Since its new, perhaps the lines are not bled enough to have constant gas flow. I've smelled gas and still had this issue, but usually on the 3-4th attempt it will light and be fine.
Even though its running 5min, if the ignition lockout is happening, it sounds like its never lighting. The blower will run for a period before and after the burner cuts on. Do you ever hear it actually igniting and pushing hot air?