So, the factors are:
Mail forwarding
Residency for tax and legal purposes
vehicle registration and inspection
As travellers we needn't be concerned with specific sales taxes because we move about and really don't consume like most citizens. Fuel costs vary but the same applies, you're gonna go through so you find the lowest price and move on.
Do all states look at the 180 days in state then require residency and vehicle registration? Seems to me it would be irrelevant anyway for a traveller, 180 days is 6 months, you're not a vagabond if you stay somewhere more than 6 months (thats a joke by the way, don't anyone get offended).
I have Hawaii residency, I only pay fed taxes on my SSI and pension. My medical, which I consider very important is based in Hawaii but is Blue cross affiliated so it can be good anywhere.
I will never own a vehicle on the mainland registered in Hawaii because an annual inspection is required. Well, it could be registered when you ship it but would not be able to re registered when the year is up because you need a safety inspection. I don't intend to ship a vehicle from Hawaii to the mainland, its just to expensive. For me, Pahrump, NV looks the best for the vehicle and mail forwarding. Insurance costs are a concern, how are Nevadas insurance rates?
As I see it, there are fixed costs and there are flexible costs and they are arranged individually... And if nothing else, we are all individuals!