Cleaning Wastemaster

bleach. then a good swill with a hosepipe. wastemasters don't have any that bleach will damage. been doing mine that way for years. 

Half or 1/4 fill it with water, pop in a bio tab, leave on floor of caravan whilst driving home. Make sure it is well secured, can't move about, and it's entry/exit points tightly sealed.

The back and forwards action of the water (maybe side to side as well) should do the trick. Then just empty it down a drain.