Rent my RV out as a business. Insurance needed?

My son has rented a couple different MH's from a place that actually rents out other people's rigs. The people provided no insurance, as such, and my son had to fill out some kind of short term insurance that he bought on the spot. My son clearly assumed all responsibility for any damage to the MH's and for liability. The place also rented trailers with the same kind of understanding.