All of the above.
Start with sanitation hoses, replace them before wasting your time on anything else. In a well set up system there should not be any dead flow areas where sewage can sit in a hose, it should all drain back to the holding tank
. If their more than 10 years old it's time to replace them anyway.
Then remove all the upholstery and either clean it yourself or have it cleaned, if possible let them dry in the sun, nothing kills mildew better than UV rays.
While it's out mix a weakened bleach mix in a spray bottle and saturate around your holding tank and anywhere the old hoses may have leaked. Don't forget to check the holding tank vent hose and vent either, especially on an older boat.
Then clean the rest of the boat hard interior
surfaces with the weakened bleach mix to get rid of mold
, mildew and any organics that may be taking hold.
I wouldn't recommend bleach in the bilge
though since it may not be good for the bilge pump
, the earlier suggestion of using vinegar is a good one.
Had to do this on my just recently sold boat when I bought it 12 years prior, since going through it that way it only required a going over once a year during commissioning to keep it smelling fresh and mold free.