I've never used the head on my 2013 2520XLD but it was pretty clear the previous owner did, probably a lot judging by the stains on the floor around it.
So here's the question: even though it doesn't smell bad, how do I make sure the tank is empty and cleaned out (and that everything still works in case we do need to use it one day)? Do I have to go to a marina and have them pump it out or is there a more DIY way to flush it and maybe run some cleaner through it. And, also, should I winterize it in some way after I'm done cleaning it out?
I have the same boat and year-model as you, so I'll guess you and I have the same electric head?... First, (imho) Parker uses one of the worst/cheapest heads. It is the only marine head I've ever owned (7) or seen that does not have a separate 'water-in'/'wet-bowl' flush setting, and a separate 'dry-bowl'/ 'discharge' lever or switch. This makes it confusing to operate, and it does not do a very good job. I would have preferred a manual system, with the proper switch levers/controls.
It is against Federal Law (and common decency) to flush black-water waste overboard any where in the USA. You must be at least three miles offshore. If not offshore, then use the pump-out facility at a marina. (I could write a book about 'The Joys Of Pumping Out a Holding Tank'.. it is one of the least enjoyable chores of boat-ownership; there is No amusement-value! ☺).
If the previous owner used the head for 'just liquids', then I understand where it may not 'smell bad'. If it was used for 'the other', and your cabin, head, hoses and holding tank do not smell bad, then I'd love to know what your previous owner fed his family. It must have been a special diet!
Yes, you have to 'winterize' the head if you live where it freezes. Some people use the 'pink-stuff' non-poison? anti-freeze. It's best to try and drain it all and/or pump it dry. Not such a big deal with the plastic/rubber hoses and plastic/vinyl? holding tank, but it is a big deal with the two pumps (yes there are two; one pumps the head, the other is the macerator pump under the pilothouse floor near the holding tank) and with the inner parts, and the porcelain discharge section from the bowl.
A lot of folks who actually use their heads (the toilet, not the gray-matter under their skulls) use the chemical head treatments (like used in campers) to help control the odors. Some of these can help, but there are a lot of variable as to how long they work and how effective they are. A lot of the problem is that the waste hoses, and PLASTIC! (holy crap, no pun intended) holding tank is NOT vapor impermeable... eventually the smell will get out....
The previous owner of our boat never used the head. We used it one time, the first week we owned it (It was an emergency). It took a lot of chemicals and flushing to bring the boat back to 'normal' after that.... We never had any head odors on a previous boat (owned and used for 28 years). It had two heads that were used extensively. They were coupled to an 80 gallon stainless steel holding tank, and super-vapor-non-permeable hoses that were later wrapped with Hardcast Foil-Ray butyl rubber and aluminum tape. Never had an issue with that system.
For added maintenance, if you're going to use the head, it's good to use a special head lubricant on a regular basis. It lubes the rubber seals and inner working parts.... Do NOT use cooking oils as a cheap alternative, as was once recommended by some. These will harden/coagulate and completely clog the system.... In a pinch it is ok to use a little mineral oil, but it's best to use a manufacturers- recommended head lube....