We purchased a shirt king mattress for our 44' fifth wheel.  We had a topper on the mattress that came with the rv but couldn't sleep well, my husband is 6'6" and I'm 5'3" and we were tired.  I saw a family post and they live in their 5th wheel full time.  They purchased ghost beds for their 4 kids as well as themselves and were super pleased.
And now we are too!