What the law requires
Regulation (EC) No 852/2004, Annex II, Chapter III sets the rules for mobile sales vehicles and temporary premises, including an adequate supply of hot or cold drinking water and the hygienic disposal of liquid waste. Chapter VII requires an adequate supply of drinking water, to be used wherever it is needed to keep food from being contaminated. In the UK, the Food Standards Agency pack Safer food, better business asks for the same kind of cleaning records. How often you clean and disinfect the tanks and which product you use come from your own HACCP plan and the national guide.
Record every time a tank is emptied, cleaned or disinfected, with the product used.