Once someone rents their vehicle to someone else, their insurance will no longer be valid. It is an exclusion in every personal auto policy I have ever seen. So, you will be taking your chances & the owner will be taking their chances if you do this. There will be no liability & no collision coverage on the camper van. You may possibly have coverage on your policy, but maybe not. You would have to check with your company for that. Still, if you have coverage with your company, that does not protect the liability of the owner. They could get sued in an at fault accident & your coverage would not cover them & their coverage would not cover them.