I can answer some of the questions, but don't know the fees, because in most cases they are taxes based on value or purchase price.
In most states now, only a resident or resident legal entity can register a motor vehicle. This is a result of states trying to comply with Homeland Security rules about positive identification. There are ways to work around this, easier in some states than others. California is not one of the easy states.
Registration of camper trailers may or may not be different. Some states (Oklahoma for one) do not even require registration or licensing of trailers under a certain size for private use. Other states treat them as motor vehicles. These differences mostly have to do with tax codes, where there are personal property taxes and whether or not vehicles are treated differently. Local jurisdictions also tax personal property, so licensing costs might vary place to place within a state.
You can use your Australian driver's license to drive here, if someone else owns the vehicle and has given you permission to drive it. If it the vehicle is licensed to you, at a U.S. address, most police jurisdictions will expect your driver's license to reflect the same address and be issued the same place. This is not necessarily a legal requirement, rather a situation that requires adequate explanation.
Insurance costs from U.S. insurers depend partly on where a vehicle is "garaged" which is assumed, lacking other information, to be the address to which it is registered. Liability insurance, and to some extent other coverages, also depend on your driving record and credit history (if that is allowed in the state where insured). Each insurer has its own rules and underwriting standards, and an atypical situation may result in higher rates, if you can get coverage at all. You can call around (once you are sure where you will register the vehicle) but in general the insurers will not quote rates for hypothetical situations; the insurer wants the VINs of the vehicles involved, and drivers license numbers and addresses for the background checks used to determine rates.
Do not expect refunds of any taxes or fees. Most states do not have VAT, nor a VAT rebate program to encourage tourist spending. Most of our taxes on vehicles are either excise taxes, personal property taxes, highway use taxes, or sales taxes on the commercial transaction. In many cases for vehicles the taxes will be a combination of more than one.
You might consider hiring a legal representative at least an experienced agent, to work out the details for wherever it is that you want to do this. There is some time vs money tradeoff, and sometimes what costs the least time is to have an agent set up a corporation to purchase, own, register and insure the vehicles in question, then lease them to you as a visitor.
Tom Test
Itasca Spirit 29B
