We have the same problem with our 21' Coachman hybrid. We cut a piece of memory foam to bridge the gap. The foam helped but did not eliminate the problem. We then tried queen blow up mattresses but didn't like the banana effect.
We are still working on the problem.
Trevor & Susie Stafford
2012 Coachmen Freedom Express LTZ
2001 Ford 250 7.3 Power Stroke