WATER AN COOLANT DISAPPEARING

Assuming you've already checked the obvious- radiator, hoses, water pump and they are OK here are my suggestions:

If your radiator system has a bleeder valve you need to bleed the system or you will keep getting air pockets that show up in the radiator as low levels of water that need topping up. Check if your engine has this and bleed it- usually VERY SIMPLE process to bleed the air.

Is your carpet/floorboard wet or damp?- heater core problem

Is your oil milky?- internal engine-head gasket trouble, cracked/frozen engine block- sorry.

Have you checked your freeze plugs?

These are pressed into the block in the water channels to keep the block from cracking in a hard freeze. These are meant to pressout of the block when the water expands during a hard freeze to give room for the ice instead of cracking the block. Corrosion and pinhole leaks can form and are accelerated by failing to use antifreeze or letting it go stale.
These get pinhole leaks and may not be obvious. You might hear a hiss if water or vapor is escaping under pressure. Go to the local parts shack and see if they have a diagram that shows the freeze plugs for your specific engine then try to get a look at them- not likely since a lot of them are in very inconvenient places.

A complete set is probably $5-10 but finding the one and getting to it is priceless. They vary in size from 3/8inches to a couple inches diameter. If this is your problem I hope it is accessible and not in a difficult place, like behind the flywheel or under the exhaust manifold or engine mounts- Good Luck!

If you can't find it just keep your water filled and eventually it will become obvious where the leak is. Unfortunately with even a pinhole leak you lose pressure, which is the greater part of what is keeping the temps down in your radiator.

Please post a rating for my efforts

This is not a good sign. have your oil changed and see if their is antifreeze in the oil. If so im sorry to say it but you have a head gasket leak. Very common on the F-150's

If you have ever overheated you may have warped your head and the coolant is evaporating away. Is is running poorly. To test this theory use a large plastic pop bottle and cut the bottom off and place the bottle in the top of your rad. Fill the bottle up and if bubble begin to perculate in the bottle when the engine gets hot then you have to get the head machined or goto the wrecker