I think I've honestly been through 10 car seats with my 2 kids between the car, van, Grandparents.
Our rock and roll bed didn't have ISOFix when we got it and I went with seatbelt held in seats to start with but they are just a pain in the arse and even though I was fitting right I never felt they were fully secure. Not to the point of worry for the kids safety as such but there was always that niggle.
Ended up buying 2 ISOFix brackets and bolting to the frame of the bed. That might not be crash tested but I can safely say the van would crumple way before the brackets gave out. Now use a joie seat for my 18 month old and my 5 year old is in a ISOFix booster seat.