Mattress Height

I have a 2007 2720SL that I'm outfitting for use on my first trip early next year. I've been working without any time off, so my unit has been in storage. I want to change the factory mattress to a thicker one. Does anyone have any experience installing a thick mattress and finding that they're having problems closing the unit? the mattress I want to install is probably twice as thick as the one that came with the unit.

Dan
2007 2720 SL
2002 Toyota 4Runner Limited 4x4
Supercharger w/ TJM Bullbar & Winch