I am putting in a service for my trailer house and my wire is to short. Can I set a box on the under side of the house (mounted to the house) and splice the wires?

Yes and that is where you'd install a disconnect. I'd put it on the side and not under the trailer, the panel short be accessible. What size/gauge are you using and how many amps will your trailer draw if evberything is on at the same time?

There is already a disconnect on the pedastel. And I'm running a 100 amp service with 2-2-2 copper and a #4 aluminum ground

Where is the pedestal in comparison to the source power the power and the trailer? Is this similar to what you are using 378' 2-2-2-4 SER Copper Service Entrance Wire Cable - Amazon.com

The pedestal is only about 15 feet away I need about 20 feet of wire to reach the panel I'm short only 5 or so feet. No I am using I think thhn wire in 1 1/4 CONDUIt

What is the gauge of the wire? I would still use an exterior disconnect box for your splice. It's already weatherproof.