I know you said you had watched various videos, but here is the one from the Truma site - I thought it was hard to find so you may have missed it. They have a site for US and one for Great Britain - the GB site has more information than the US site.
This page has links to three videos and you want the operating one (#3), obviously.
https://www.truma.com/us/en/products...combi-eco.html
Also, there is a 12v fuse inside the heater that could be blown, but would need to be replaced by a dealer to maintain warranty.