In Ca what are possible penalties for driving a friends RV with expired tags not renewable because owner defaulted on loan and repo attempts have failed and owner is involved in lawsuit with bank?

Not sure about fines, there may be some which an operator must pay for driving a vehicle without current registration. Biggest concern I'd have is that the RV would be impounded on the spot with all personal possessions on board. Getting even clothing back might take a lawyer, and the driver/passengers would have to find other transportation immediately.

Might be cheaper in the long run to just rent one that is legal.