The tank on as 2wd short bed truck is just under 14 gal. The long bed and extended cab trucks have 18gal tanks.
It does sound like coolant is leaking into one or more cylinders. Try this: run it long enough to pressurized the cooling system (10-15 min) and it smooths out. Let it sit overnight then start it for just a few seconds. Remove the spark plugs and look for ones that are very wet. If not sure or more than one is wet, position a piece of cardboard over all four plug holes. Turn it over a number of revolutions then check the cardboard for fluid ejected from the cylinder(s). This test worked for me revealing #4 had a coolant leak.
If you find coolant is leaking into a cylinder and you need to run the engine or drive it, loosen the radiator cap after each shutdown. This will relieve the cooling system pressure and reduce the amount of coolant that will seep into the cylinder. Just use a towel over the cap so it catches anything that sprays out rather than on you.
Good luck