Larry is right, it is not too complicated.
I tried something new. I drained and flushed the black tank. Then I ran a hose through the bathroom window, poured a gallon of bleach in and filled the tank. I filled it so full that the toilet bowl was half full. I poured in another half gallon of bleach so that the work area got a shock treatment of chlorine. Then I drained the tank. I don't know if I really killed the "germs", but all of the odor went away. If you hire someone to do this, do this as a favor to him.
Getting the right parts is a little tricky. The parts list Beaver pasted to the bathroom cupboard door has the model and serial number. The Sealand Dometic website suggests you call with the numbers for parts identification. I didn't. Instead I went to an RV parts store in Las Vegas. I spent $100 on two kits. One had the two part seal and the other had the ball and spring mechanism. I did not get a new water valve because it wasn't leaking, is on the outside and replaces with two screws and a hose clamp.
The seal kit was for toilets made after 2001. Apparently my 2004 Beaver by Monaco was built with old parts because my new two part seal was missing an "over flow vent hole". I borrowed a roto saw from Mike Humble and sort of melted the extra hole in the two seals.
I was luckier on the other kit. The parts guy said the spring assembly was usually white, but sometimes black. I went with the kit that had a white one and it turned out to be correct.
If you are going to do this, do the whole thing. Replacing the seals is easier than the ball. But, the ball grows hard water mineral deposits and develops scratches over time. The seal won't seal against a rough ball and will wear the seals faster. My original seals lasted for 48 months of continuous use. The second set lasted about 12 months of continuous use because the ball had developed a rough surface.
If anyone is interested, I took video of most of this process. The only tricky part is actually getting the old ball out and the new one in. There is only one screw, it is in an awkward spot and both hands are required. I was too lazy to set up a tripod, so that part is missing from the video.
Start to finish it took 3 hours. Cleaning the tank, gathering a few tool and rags, borrowing a tool from Mike, modifying the seal and cleanup was most of it. I was only on my knees with my hands in it for about 30 minutes. My reward for a good job came from BJ who took me to the In-n-Out for lunch.
