The best way to get rid of mildew is to remove ALL cloth and upholstery from the boat. The cushions can go out for professional cleaning (they steam right through them) or if they are that old, it is probably worth replacing them outright with new. If there's a headliner, it needs to come down.
Bleach rinsing all over the interior will kill mildew, but it will come back if the boat is not keep dry and ventilated. About the only way I know to assure that is to install the solar vents, which will do a better job than leaving open anything else.
Remember, you don't want to get rid of the mildew SMELL, you need to get rid of the mildew itself, then the smell will also go away. Once you've got it as clean as you can, you might want to close up the boat and run an ozone generator in it for a week or two. The ozone will penetrate every space and kill the rest of the mildew. Concentrated ozone also isn't good for human beings and it makes rubber and plastic brittle after extended contact, so you'd want to do that as a one-shot and then use something like ArmorAll on all the plastic afterwards, to counteract some of the damage.
There are also mildecides and "quaternary" (sp?) agents that will prevent mildew from coming back, but if you keep it well ventilated you shouldn't need them.