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.