Depends what kind of toilet you have. But usually you flush from inside your van until no more water comes out, then open toilet hatch on outside of van, somewhere near the top you should see a rubber hose clipped in. Pull down this hose, and place a container near end, then remove rubber bung. You will get a good egg cup more water out. One furthe rpoint do not forget to drain your shower, water will collect in the bent bottom part of shower hose. Unscrew hose from mixer valve bit and blow down end. You will be surprised how much water comes out. If you fail to do this the water in curved bit of shwer hos ecan freeze.
------------- It's not what you take when you leave this world behind, it's what you leave behind when you're gone
Depending on model some pumps that were installed in the bottom of the cistern under where the cassette slides in did not drain properly and froze.There was another modification drain tube available for these, but if it has not been done you need to also remove the small braided plastic U shaped tube to release the trapped water in the pump.
Forgot to say Rusenil- you may find two rubber pipes , if so same applies, just drain each pipe. Thetford did a mod a few years ago as they found the tolilets were not draining completly, hope fully yours will have been done. They supplied the parts for diy fitting , and it was a straight forward job.
------------- It's not what you take when you leave this world behind, it's what you leave behind when you're gone
When flushed all of water put egg cup of car window washer anti freeze in tank wont effect pump rubbers. I have used anti freeze for years in flush tank and soil tank when vanning in frosty weather.