This depends on the model of grill you have selected. Many modern portable camping grills that utilize the smaller 1# propane tanks feature specially designed holder brackets for the tanks, so they neither get in the way nor put any gravity stress on the propane line. Larger grills on the other hand, grills that are designed to be used with full-size propane tanks, cannot support the weight of so much gas. For these grills the full-size tank is kept on the ground, usually beneath or behind the grill itself.
SAFETY TIP: Remember that gas leaks can be dangerous, especially when using a full size tank. Checking for gas leaks in the system is quick, easy, and can keep you safe from unwanted fires. If you are doing a lot of cooking, it is a good idea to include a fire extinguisher in your gear kit.