I've been in contact with the California ARB. It has not been very helpful, but I'm not done yet. The problem is that they are all about electric and hydrogen fuel cell vehicles. They don't pay much attention to big old motorhomes.

As to the valve seats, I don't think that is a worrisome issue. There are a lot of fleet vehicles that run for a whole big bunch of miles on natural gas. Oh, and in the US, natural gas == propane. In Mexico, you are likely to get butane. Different stuff, so be prepared. They do not interchange. It is the equivalent of gasoline vs. diesel fuel.