yes the old double gasket inside the tank nozzle problem
thats one thing to check

here are a couple of other things to check also
a) make sure the ignition is getting spark at the spark plugs, pull off a wire and try to start the engine, with a screwdriver stuck inside the plugwire and held close to the engine block see if you get spark.

b) insure you have ample air flow, check the air cleaner and associated hoses to make sure they are not clogged up with dirt. removing the air filter temporarily should suffice.

c) identify if it has an electrical fuel lockoff or a manual one
if electric make sure its working by first turn ignition key to on but dont start the truck and listen for a click, if you hear it click then thats a good sign, BUT also you can loosen the fuel line AFTER the fuel lockoff at the inlet on the vaporizer and briefly cycle the keyswitch and see if you get lp gas there. YES there will be a loud rushing sound of gas so just cycle the key very briefly. IF you get gas there this has eliminated everything upto the vaporizer as being the problem.

BUT....IF it has a manual lp filter/lockoff which is a separate round metal device usually located right beside the vaporizer and this is where the fuel inlet is, check the vacum hose to it, this usually comes from the mixer body and hooks to the vacum port on this.... if the hose is damaged or unhooked the truck will not start because it is not being opened from the engine vacum while the engine is starting.

oh and last but not least, make sure the lp tank has gas in it, you dont know how many call's i've made in the past where it was just a simple thing of the tank was just out of gas. (of course i'm sure you have done this first off ;o) )