There are two companies in the Washington area that build trailers for boondocking and rough terrain. We considered buying an Artic Fox but they are few and far between in Texas and we just weren't willing to drive that far to buy something unknown. We also don't have quite the boondocking opportunities that you have in CA either. But profdant139 is on the right track with his modifications and I think there is a market here for that.
Look at Allen Manufacturing there in OK. They build a heavy rig, without all the bells and whistles that people have come to expect, and they have a waiting list.
Something to think about if you can build quality.