Answer: one of the mail services designed for RVers (there are several). We are Escapees and use their mail service and other benefits. The domicile issue for the Escapees mail service has been tested in the courts and stood. We do all our banking and insurance with USAA with no issues at all. Last year we applied for TSA's Pre-Check using our Escapees address and had no issues with that, either. Most recently, we are buying a new RV and applied for a full-timers loan through Essex Credit (Bank of the West) and that was approved in a couple of days.
Rob