All marine grade Toilet paper is is a cheap single ply toilet paper. Any toilet paper that starts to disintegrate on contact with water should do nicely. Scott Tissues makes a good toilet paper that works well in most marine heads, and it's a far cheaper alternative than the "marine" brand stuff.
The Joker Valve is usually a large flapper type valve that is supposed to prevent the waste from backflushing into the head, back through the pump. It is usually pretty simple to replace and is available as part of a re-build kit for most heads. I'd also replace the pump seals, since you'll be tearing the pump apart, and the seals you need will also be in the re-build kit.
However, depending on what head you have, it may be only a small price difference between re-building it and replacing it... so you may want to check that out. This is what SailorTJK has done.
Most marine heads are rigged to take water from a through hulll and use the salt water to "flush" the toilet. This then gets pumped into the holding tank or out to sea (if out past the three-mile limit).
A holding tank is never called a freshwater holding tank. If it is a holding tank, it is a black water holding tank--if it is freshwater, as opposed to sewage, then it is just a water tank.
Some heads are plumbed to allow you to flush them using fresh water. That is an option with the head on my boat. The intake line to the head can be fed from either a through hull with sea water or from the drain of the head sink, with fresh water. Dead sea water organisms are often a big cause of head odors...so flushing with fresh water might be a good idea to cut down on the smell.