Found this in another thread, in case anyone else was wondering the same thing.
Here is an inline coolant heater from Northern Tool, it's not a 240 volt but it does a great job at 125 volt.... easy to set up for $50.00.
Kat's 1500 Watt Circulating Tank Heater — Model# 13150
http://www.northerntool.com/shop/tools/ ... _200339137Overview
Thermopump heater is designed to heat and circulate the coolant in an engine for easy cold-weather starting.
Has a thermostat and extra-long cord to connect to AC power source. U.S.A.
Features + Benefits:
Heats and circulate engine coolant
Aids cold-weather starting
Made of die-cast aluminum to resist corrosion
What's Included
(1) 1500 Watt tank heater
Key Specs:
Primary Heating Application Vehicle
Volts 110
Watts 1,500
Thermostat Included Yes
Power Cord (ft.) 4
Remote Included No
Manufacturer Warranty 12 months parts / 12 months labor
Ship Weight 3.0 lbs
Northern Tool Item# 174050