I have the installation manual for my Atwood water heater, but it's too large for this forums 2MB pdf limit. It has a standard foam outer shell. Page 2 indicates you can do a zero clearance installation, so you should have no problem doing what you propose. Im attaching a copy of page 2 from this manual.
Hope this helps,
- - Mike
2013 Airstream Interstate Lounge EXT on 2012 Sprinter 3500 170Ext
Formerly: 1973 Dodge B300 DIY pop-top conversion