We use fitted twin sheets and the mattress straps but we also use the vinyl sheet protectors. All of this wrapped around the mattress. Then the kids use sleeping bags (just the cheap ones) after all it is camping. This way the kids are just dirtying up the sleeping bags and if there is an accident with soda or other bed involved liquids we just pull the sheet off wash it with the sleeping bag wipe down the vinyl and there you go, clean unstained mattress, and if you count the cold adult drink at the laundry facilities not a bad way to spend some alone time.
With a wife (of 15yrs), 2 kids (15,4), 2 dogs (17,1 human years), House payment (way to high to mention), trailer truck car payments, and I'm an engineer my life is filled with drama the easier the camping experience the less the therapy bills.