Need some grease and a cheaper option on the strip? You need to try a Fatburger! They have really fresh ingredients for a fast food joint. They have some decent milkshakes as well. There are a plethora of options that you can add to your burger as well. Check out the menu and mix and match something that would fit you!
Don't waste your time in McD's or any of the other fast food places. The only fast food restaurant I'd recommend in Vegas is In N Out but that's off the strip.