Siding needs repair (Wood and Fiberglass)

Unless you are VERY good with fiberglass, and I mean REAL good. Take it to a pro. Let some one that knows what they are doing, do it.

I'd do the same as Winged says but not do any leak control until the glass work is done.

FWIW, I sealed up my siding and roof edge trim with Geocel Tripolymer and Eternabond on the roof. All is leak free, now. I believe the standard sealing method for that corner trim is a butyl tape positioned between the siding and the inner corner edge trim. I'd seal up the corner with Tripolymer before putting on the butyl tape, extra insurance. BTW, any rotting wood is not going to get better, it will eventually dissolve into powder.

Thanks, wing_zealot for the suggestion. The Trailer is paid for and we only use it a few times a year so I agree it's probably worth the effort. Thanks for giving me a good order of operations.

This is what I would do.
1. Fix the leak
2. Remove the damaged wood and replace it with new wood
3. Bond the fiberglass to the new wood (same as you would do for a countertop)
4. Patch the missing damaged fiberglass (just like they do a Corvette body)
5. Paint to match (or should that be pain to match?)
That is an overly simplistic explanation for something that is going to be a pain in the shorts. But it's not impossible, and is probably worth the effort.