You home was not designed to sit on a perimeter foundation and adding one would do little to help support the structure.
If you are not having any level problems then just watch it and dont worry too much... the homes been there for some time.
If you are having a problem and think the home may be out of level call three of four licensed manufactured home installer and get competitive prices for releveling and if necessary adding additional piers.
The operations involved in jacking and raising the home can be very dangerous and should be attempted only by experienced individuals. You can test to see if your home is out of level. the Releveling Kit from Aberdeen Repair
includes complete step-by-step instructions and a waterline level. Use these instructions and the enclosed waterline level to determine whether or not your home is out of level. You will also use this information to become a better-informed consumer and give yourself the upper hand when it comes to hiring a contractor to do the job.