I am the same way... but I never worried about extending the base just put the larger mattress on after all its only your feet there... oh I never have never sat on the edge of a bed so that might affect things too.
2004 Chev Silverado Duramax optioned past the max. 2009 Jayco Eagle 308 RLS 765 watts of solar, 6-6 volt batteries (696 amp hour), 2000 watt (4000 surge) whole house inverter.
140 days boondocking in 2016
211/2015, 196/14, 247/13, 193/12