No idea as I've never been but is it one of those countries where you get vehicle insurance at various booths as you enter the country? Quite how good they are is anyones guess...unless it is for actual travel insurance covering you riding a motorbike in Tunisia (I don't know the answer to that one either)
Maybe some info or links here
FCDO travel advice for Tunisia. Includes safety and security, insurance, entry requirements and legal differences.
www.gov.uk