In some countries (including UK, EU, Australia, NZ & most of Asia) it is compulsory for the car owner (i.e. the rental car company) to obtain third party insurance. In the US the responsibility falls on the car driver. There are also some differences between third party property (damages to another vehicle) and third party personal (injuries to another driver or their passengers) from region to region. Lastly, there is often confusion in Australia and New Zealand because rental companies decut third party damage costs from the excess that is payable in the event of a multi-vehicle accident.
In Europe third party insurance (usually called "civil liability") is compulsorily held by the car owner and it provides coverage to damages to other vehicles and injury to other drivers. Without it, a rental company cannot place a vehicle on the road. It is consequently not sold by the rental companies, nor is it offered independently by RentalCover.com.
Read this FAQ for more information on the European insurance options.
In the US and parts of South America, third party responsibility lies with the driver of a rental vehicle. In those countries you will be offered Supplementary Liability Insurance (called SLI or LIS) at the car rental desk and sometimes you will be offered SLI online from the rental company's website. SLI provides coverage to car drivers who cause injuries to drivers or passengers of another vehicle (called "third party injury" or "third party personal"), and other vehicles that are damaged by the accident ("third party property"). Some US states mandate that all cars including rental car companies include SLI however the state minimums are usually quite low for third party injuries so you may consider purchasing an SLI policy. At present RentalCover.com does not sell SLI policies.
In Australia and New Zealand it is compulsory for vehicle owners (i.e. the rental company) to have insurance for third party injuries so a rental vehicle cannot be registered or rented without it. In NZ third party personal injuries are settled through a government program called ACC which does not ascribe "fault" to either driver.
Regarding third party property (damage to other vehicles), some motorhome rental companies charge their customers for "third party" damages but they are just discretionary excess charges, i.e. it is a way for them to increase your damage costs. RentalCover.com policies do not discriminate between one type of excess charge and another, all are insured. So yes, third party damage costs are insured. But we suggest you read our detailed explanation of "third party" charges by motorhome rental companies in Australia and New Zealand.