Well yeah finding a tank would be troublesome. But considering we live on the coast I bet it wouldnt be as hard as you think.
Or hey. You could even scrap the tank idea and go steal a cruise ship and just sail for a couple weeks. You'd still eventually need to refuel. But at least you wouldnt be bothered while at sea. Only trouble you'd have is clearing the 500+ rooms of zombies before setting sail.
But I guess the ultimate fact about this is that if a zombie outbreak ever happened. Noone would survive. Even people with military bomb shelters with TONS of food would die eventually. Our only hope would be a cure.