If you install the tank type heater inline on one of the coolant hoses serving the LP vaporizer/regulator, you will not have any air lock problems.
The heat will cause a natural flow of coolant to occur via convection current. You just need to mount the tank heater at the lowest point you can in one of the hoses and have the heater angled upward to induce the best possible convection.
Mounting the heater low exploits the fact that heat will cause the heated medium to rise away from the heat source upper end and so cold coolant will be drawn toward the heater lower end as the heated coolant rises and displaces coolant above in the process.