There are several places that I know of where water can enter through the step. Try to determine the source.

1. The four through bolts that secure the step plate to the coach roof. If the sealant around these has failed water can enter and drip down the compression post. If that is your source i suppose you can clean around them from the inside and apply a good sealant to stop the leak. No guarantees, however.
2. Same goes for the short piece of conduit that extends through the coach roof. If the mast electrical wires that enter this conduit do not have a drip loop before they enter the conduit rain water will follow the wires through and into the cabin. The only way I know of to create a drip loop is to lift the mast up high enough to make it, or I suppose cut an access panel into the mast.
3. The sealant around the conduit that enters the coach roof has failed. You can try to clean out as much of the old sealant as possible and apply new and hope that it stops the leak. No guarantees here either. The PO tried to do this using Silicone II from inside the boat. What a mess and it still leaked.
4. The above items result from rain water coming down the inside of the mast and pooling inside the step plate. If the plate to deck seal has failed, water can enter from outside to the inside of the step. You can clean and reapply fresh sealant, but again no guarantees.

I watched every step the operators went through to un-step and step our mast while the boat was on the hard. If there is a fishermans dock close by you can do this while the boat is in the water. This should be done by professionals, but it was something I felt I could do myself in a pinch.

If it were possible I would rent a small crane to lift the mast high enough off the step without taking everything down. Two people minimum; one to operate the crane and one to guide the lift from the step plate. Tape around each stay turnbuckle to mark the current tension before loosening. Wrap the crane lift strap just beneath the upper spreaders.

The mast needs to be lifted about ten inches to give you enough room to work on the repair. Unless the electrical connections inside the mast need to be repaired, I would leave them alone. Just follow the rest of the steps that I detailed in my Knowledge Base post. One day should give you enough time to do everything.