Re: expansion joints
However, if frost lifts the slab on which the mobile home is mounted you will have a few problems besides a broken conduit.
You do not want to bury an expansion joint in the ground. In at least 1 mobile home park around Akron, Ohio tree and grass roots have penetrated O-ring gasketed joints in PVC sanitary sewer pipe. A PVC or steel conduit expansion joint also has similar O-rings.
What I would do is to put a sleeve around the feeder conduit where it emerges from the slab so that there is no solid connection. I would then run liquidtight nonmetallic conduit from the rigid PVC stub to the conduit that goes from the underside of the mobile home to the pnaelboard. I would also use a PVC pull box between the rigid PVC and flexible PVC conduits. Flexible PVC conduit has a very high coefficient of friction.