Update on the Webasto. It might be fixed.
Yesterday I contacted Webasto Tech support and asked if I could send my heater to them. I wanted to make sure all of the components were operating properly and have them reset everything to factory specs. An hour later I got a call from one of the techs. This was a guy I hadn't talked to yet, he told me he's been working on them for years and years and that he knew a few tricks.
He told me the gas version of the airtop can be more finicky than the diesel version.
His advice was to pull the unit apart again, clean the glow plug with emery cloth and look for a small hole under the glow plug. If it's clogged the clean it out. This small hole passes air over the glow plug and helps it start combustion. He gave me his cell phone number and told me to call him at home when I had the heater apart. Excellent customer service, above and beyond what I expected.
When I got home there was a break in the rain so I pulled out the heater. I got it out just in time; it started pouring rain as soon as I got inside my shop. I've pulled the heater apart a few times now so I can do it pretty quick. When I got the glow plug out and down in the recessed area under where the plug sits I saw the little hole, completely plugged up with black carbon! I hadn't removed the glow plug when I took it apart to clean it last time, that was a mistake.
Here's where the hole is -
There was another brief break in the rain so I ran out to the van and bolted the heater back in. Sure enough, it fired right up! I ran it for an hour then shut it down and let it cool off for a while then tried it again and it started right up. Same thing this morning, quick start, no fumes or gas smells. I can't wait to try it up at the pass this weekend.
I wish I would have known about this months ago. I should have removed the glow pin the first time I cleaned it, oh well. Live and learn.