Which mail service are you using in SD?
The mail service 'address' is a legal physical address.
You have a 'contract' with them for your residency.
Do you have 'full timers' insurance on your 'home'?
We had 'home' insurance (same type of policy that was on our S&B House) on our 5th wheel as it was our home.......property liability//casualty insurance plus.
Unfortunately lots of FTs have had issue with MEDICAL INS. Companies since passage of ACA (Obamacare). AND they (medical ins. co.) have their own set of rules as to what they will accept as proof even though State/Federal/IRS etc. accept/acknowledge the FTrs 'physical address'