Why wont my 1979 460 ford start?

Why wont my 1979 460 ford start? My RV stalled while turning it around. I thought it was out of gas. It has two tanks. If I dump gas in the carb it will start for a second but does. I have fuel present before the carb. I checked by disconnecting and putting a rag over the line. It doesnt seem to make it in the fuel bowl. I'm new to this and I'm hoping someone can help.