When you have the filter canister off, wipe the bottom of it clean with a paper towel, or blow it clean with an air compressor nozzle. Typically the particles you see sitting on the floor of the canister are too heavy to be "swept" away by fuel flowing out the drain tube, and in fact, if you look you will see that the drain tube exits the side of the canister, leaving a small amount of fuel at the bottom anyhow.
Refilling the canister as much as is practical before reinstalling it will greatly reduce the amount of time it takes for the LP to finish filling it before starting.
Put assembly back onto the truck and bump the starter 1/2 a crank, just enough to engage the LP for 20-25 secs. You will hear the LP running. The sound will change slightly when the canister is completely full. If you try to start it before the canister is completely full of fuel, it will run for about a second, then die and you will be cranking for a loooong time to get it going.
Tim