The materials use constructing the wall will determine the separation distance required between the wall & back of the cooking equipment.    In mobile food units, space is a premium and this distance is a minimum.   Then under Mechanical, Fire, or  Gas/Electrical Codes the wall construction is required to be ‘non-combustible’.       This reference from HD of Minnesota (another thread) demonstrates this requirement.  Check your state Codes.  http://www.health.state.mn.us/divs/eh/food/license/summary.html  … download and look at page 28 of the ‘Food Service Construction Guide’
 
There are a couple of threads that already discuss this problem and members have stated their use of plywood covered by metal (SS) sheet.   Be aware, wood will char if held at a constant temperature of 200 deg F; and a 1906 building fire was proven to have started from the constant contact of 170 deg F water pipe causing auto-combustion of the wood stud.