Rick - you're still referred to as "boat buddy" at our house because of all the help you gave us when we first bought Eagle.
Rachel, forgive me, I live in Florida and sometimes forget that people live in places (voluntarily even) where the water gets hard in the winter. Yes, pretty sure there is still foam inside that rudder, so it probably would be a good idea to drill and drain it if it's flooded to prevent expansion cracking. But it shouldn't cause any corrosion issues, which is one of the biggest sources of rudder failure. While we're talking rudders, you should have an emergency tiller, looks like a piece of pipe with a bend in it, in one of the cockpit lockers. Check to see if it fits the rudder stock. They apparently didn't always do a good job at finishing out the inside of those composite rudder stocks - we need to take a dremel to ours to remove some stray glass so that the emergency tiller will fit.
As Rick says, pretty simple to get to the holding tank, just remove the screws from the teak battens that hold the rear bulkhead in place, and remove the bulkhead. You should see your holding tank in the aft starboard quarter, and the fuel tank in the aft port. If your tank is of the standard size, it will be narrower than the one in Rick's picture, but probably a bit longer fore to aft, and trapezoidal in shape (sort of a triangular tank with one point cut off.) The fuel tank will be a mirror image. On our boat, the holding tank is just held in place by a wooden batten on either side of the tank that's screwed to the fiberglass liner underneath, but if your is aluminum, it might be mounted more like our fuel tank, which has an angle bracket welded to the inboard and outboard sides and screwed to the fiberglass liner. If you open your starboard lazarette and remove the access cover at the bottom, you should be able to shinge a flashlight in and see the tank and make out how it's mounted. There will of course also be several hoses attached to it. If you decide to pull it and remove it, or the fuel tank, I recommend removing the rear cabin door first. The tanks will just barely fit through the door frame with the door removed.