i wasunder my truck a while back and i saw that stenciled with white paint is a big 45 does this mean some time down the line someone put a 45 gallon tank into it?

Possible I guess.. but I seriously doubt it.
Unless the tank takes up the entire under-bed area, it's more than likely just a random number from who knows where or when. I mean, my truck has a 22 gallon tank.. and it's no little baby, lol. But still possible... Have fun fillin' 'er up.