nobody has mentioned another common problem that happens with LPG trucks.
I hate to mention this but it does happen, if the truck ran fine before the tank change and all of a sudden it wouldn't start back after changing the tank i would check a couple of things.
When you screwed the nozzle back onto the tank does it screw all the way back up onto the tank? Check inside the nozzles and make sure there is only a single round oring and one square dust seal ring in the tank nozzle and look inside the female nozzle on the truck and make sure nothing is stuck inside it, like a square round dust seal from another tank that might of came off and gotten stuck up inside it.

That would keep the nozzles from **** all the way back up together and therefore would not open the valves on the nozzles when screwed together. Therefore no LPG or not enough is getting to the system and it would not run correctly.

I know this seems very trivial but i've seen it happen too many times to NOT mention it.

of course if your getting liquid at the fuel line at the mixer then as others have said, check the vaporizer and filter lockoff.