My propane truck ran out of gas , how do i refill it?

i just started it up for the first time in about 6 months , ive never owned a propane vehicle before , but anyways i had it running for about 5 minutes and it died on me , the tank reads that it still has a 1/4 tank left in fuel , so im not sure if im supposed to turn something else on , i made sure the main valve was open , and i know that everything is threaded opposite , so hopefully im just an idiot , but if it turns out it did actually run out of gas , how would i refill it , and what would i need to do it?