Taxes are paid where you title and register the RV. Depending on your home base you might want to consider Montana or Florida for registration. Sometimes it can save you a lot of $$$$ by registering in a different state.
Many states allow you to specify that the vehicle will be titled elsewhere and do not collect sales tax at the point of sale. California and Indiana are exceptions to that, though. Some states of registration will allow you credit for sales taxes paid elsewhere too.
Since vehicle sales taxes are usually collected at the time the vehicle is titled & registered, you pay the taxes then.