As far as getting "crud" in that tank, I've seen some nice build up in the open part of the back of my plastic plug, so I can only imagine what's going on in the tank.
Short of having one of those nifty rinsers, I just remove the plug to drain whatever's in there, (don't re-insert the plug) connect city water and let 'er rip. I don't worry about the water coming out of the drain hole, I just let it do what it does. I don't know exactly how effective it is at agitating any buildup and getting it out, but it's all I've got at this point. I do this about every other time I have the trailer in my driveway, and I always store the tank "empty".
2014 Jay Flight 28 BHBE
2015 RAM 2500 6.4L HEMI, Tradesman 4x4, 3.73
Blue Ox SwayPro (BXW 1503)
Upgraded from an REI internal frame backpack and a Eureka 1/2 dome tent!