Not that we've ever tried it, but I would think that the power washer would be a BAD thing to use with the black tank. As Gene notes, splashing would be an issue (ecchhh), and the pressure probably would not be good for the tank components.
DW is the black tank cleaning wizard. We had a bit of an odor issue with our 2363 not too long after we purchased it. Lisa did the research (much of it here on the Sunline site), and ended up using Happy Campers Extreme for an initial treatment. That stuff is absolutely amazing. She used the regular Happy Camper at the conclusion of each outing for about a year and a half; now we use it a few times a season.
Ditto on what Mainah says about using ice in the tank. Until it melts, it really helps to break up anything that's clinging onto the tank walls.
The best thing we did was to purchase a Camco holding tank rinser with gate valve. Here it is on Amazon: http://www.amazon.com/Camco-39062-Fl...+flush+fitting
This widget is worth its weight in gold. It allows us to back-flush the tanks after we dump them, using the non-potable water supply that's found at nearly all dump stations. All you need to get is a spare water hose (we keep a 50' one that we use only for this purpose) and you're good to go. The Camco lets you flush the tank and then also the drain hose, which is nice.
Keeping 5-10 gallons of water in the black tank to slosh around while in transit is also a good practice.
Matthew and Lisa
TT: '02 T-2363;
TV: 2012 Ford F-150, 4x4/Off-road, 5.0L V8, "Big Gray"
Reese Straitline w/ dual cam