After putting two gallons of gas in my truck, why wont it work again?

I just recently got my grandpa's old 1998 nissan toyota frontier after it had been in the shop for several weeks. It worked just fine for almost a month until i waited to long to refill it with gas. I finally put a gallon or so in from a canteen, and after jumping it with my mom's car it decided to work again. Well, when i tried to start it the other day the battery said it was dead again! Now it wont work again and I don't know what to do. If anything is new about the truck it's that freakin' battery! We got it just before buying the truck from my granfather. So...I'm not sure what to do. Should I bring it to my mechanic again even though it'll cost money to have it towed there? Or should I try and refill it with more gas in the hopes that it might work again?