What's the best way to protect my 500L tank from frosts this winter?
its not really the tank that is the problem the volume of water wont freeze, its all about the pipe work and the hoses etc
i put a oil filled radiator in the van and just throw a old duvet or blanket over the tank, block of the grill in the bulkhead, usually with the duvet and this keeps the heat in the rear of the van and everything is good to start.
i would say get a L5 or similar water heater it makes a big difference poles and hoses