In the early days of VO conversion it was quickly learned that some form of heating is required in the VO/PO fuel tank to ensure that the VO/PO is sufficiently liquid to flow easily from the tank and through the fuel lines.
Some early attempts involved using coolant heated coils of soft copper line, cab heater cores, and automatic transmission coolers to heat the entire contents of the tank. These primitive tank heaters were inefficient and so required long periods of time after the engine was creating hot coolant before the VO/PO was sufficiently liquid (except in very warm climates) to flow easily from the tank. They also presented some problems with coolant leakage into the VO and water damage to the IPs of early conversions.
More efficient methods soon developed including coolant jackets covering the entire base of the VO tank and coolant heated probe type fuel pickups...the latter proving most efficient at quickly producing ready to flow, fully liquid VO/PO. In conversions designed for WVO an efficient tank heater/fuel pickup is even more critical.
Some research and development has been done on 12vDC electrically heated fuel pickups but initial indications have been that these have the same drawback that electrically heated filters have. They draw too heavily upon the limited extra onboard electrical production which may be more useful for heating purposes elsewhere.