I planned many moons back to install a IIRC 'Webasto' fuel heater? It run from the diesel in the tank and you will find them on BMW's (breakers yard?) and other cars that have super efficient diesel engines. There is a generic kit about that uses a timer and so forth. Perfect if you are away from mains supply (different country?/wild camping?). They have a small pump that pushes water through heater system and through the block. An alternative is for those with Auto's, they can have a remote starting or on a timer, obviously not recommended for a manual vehicle, just throwing out musings from my past thoughts and ideas.
regards
Dave
regards
Dave