If you use your existing cable from the service disconnect to the mobile home panel, you will be fine. It should be 4/0.
If you don't want to use 4/0 cable, then you need to install a smaller breaker in the service disconnect.
The first option assumes that you have a main breaker panel in the new mobile home with a 100 amp breaker installed. If the mobile home has only a main lug panel (that is, one with only branch circuits and no main breaker), then you must go with the second option.
The general thing to remember is the purpose of a circuit breaker is to protect wiring, and must be sized appropriately for the wiring, NOT necessarily for the load. In your case, the main disconnect protects the service cable between the main disconnect and the mobile home breaker panel. It can be smaller than the ampacity of that cable, but not larger.
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