I'm in agreement with ImOld on this one. Water heaters haven't changed much in the 40 years since your MH was built--it was designed to handle the weight back then, so what does it need to meet the challenge today?
Is the space the heater inhabits only supported by one cantilevered outrigger or will adding a new layer of 3/4 ply span 2 outriggers? Is the HWH located in its original spot or has it been moved? Having the wire come through a hole punched in the wall sure doesn't sound original-spec to me.