If one hasn't had a boat with a fixed head with a holding tank or an RV, there can be some trepidation about its use. We have several decades of experience with these, and each type has a "best use". We do use a holding tank chemical, Thedford Campa-Chem Green (available at Wal-Mart in the RV supplies). No formaldahyde. It's available in liquid or the small packs that you toss in. It does more than keep the smell down, it also helps break down solids faster. We also use toilet paper made for RV/marine toilets. No, it's not as plush as the stuff at home, but it does break down faster.
We have found that using raw water (sea water) to flush causes more smell than fresh water. It seems the little critters in the sea water die in the lines and add to the stink. Our CD-25 has the head that uses a fresh water flush. As has been mentioned in a post above, we keep a bottle of fresh water in the head to flush with... it uses less water than the flush pedal on the toilet.
If your toilet is the type that uses sea water to flush, a fresh water rinse will help keep the smell down. A thorough flushing when you have plenty of water available can also help.
It seems that some types of hose get a smell to them after a few years and the only thing you can to is replace the hoses.
After pumping out, we put a small bit of fresh water in the holding tank, then add the chemical above. There may be a slight smell when the toilet is opened, but it shouldn't be obnoxious and should dissipate quickly, with a window or vent open.
Having worked in a marina, I can tell you there IS a difference in the smell with the folks who use a toilet chemical and those who don't.
Hope that helps.
Best wishes,
Jim B.