Back hurting from bed in travel trailer - need advice please

I have a standard residential mattress in my rv. It hangs over the plywood about six inches at the bottom. The standard rv mattress is too short for my liking. My feet hang over the bottom of the mattress. This is the second rv that Ive had the longer mattress in. No problems other than it cuts down on the space a bit at the bottom of the bed. If you are six feet or taller, the standard rv bed sucks, unless you like your feet hanging off the mattress.