Insurance is not needed because it is not a motor vehicle. It is not powered by a motor, it is only a towable trailer/vehicle. Also any sales tax you paid at time of purchase in another state is deducted from and sales tax you have to pay in California, if you have the receipt showing that you paid some sales tax.
If the California sales tax is $1200 and you paid sales tax of $550 in the state of purchase you will owe $650. Defiantly get a title
to make it easier to sell either in state or if you sell it out of state.
Sorry that our state has to tax us excessively to the benefit of untaxed others. Just a word of warning, it is coming to your state too.