How do you share group expenses without arguing on the third day?

We are organizing a road trip in four: two people drink little, one wants single rooms when possible and another proposes to put everything in a common case. In the past, small payments accumulated and in the end no one remembered what was personal. I would like to first establish a rule that remains simple during the trip, without turning every dinner into a accounting sheet and without paying to all individual choices. I am interested in a comparison based on recent experiences: indicated travel period, what rea…