thickness of cement runners

Sorry, no idea, no mobile homes in this Island.

It may be best to go visit some mobile homes around your area, and ask for information about the cement runners.

Contact your local building code enforcer... he'll know. Any information here is only hearsay and if your code enforcer disagrees with the answer you get here,... ummmm... he wins.

IT depends upon soil conditions and loading factors. Some mobile home manufacturers will give a suggested footing design. Usually they will need to be past the depth of the frost line for the location. Local inspectors will give you some information but usually the owner (contractor) has to provided their own approved desing.