A shotgun approach to repairs can really rack up the bills.

If your symptoms are loss of water and you have replaced the thermostat, you more than likely have an issue with steam. And that steam more than often comes from a bad head gasket. Most head gasket gasket failures seem to be confined to the water side initially, but let it go on bigger damage will occur.
You can buy a litmus test strip to dunk in the coolant. It will indicate if there are exhaust residuals in the water and thus a gasket failure.

However it may just be something simple, given that you have just about replaced everything:

have you given the radiator a good hosing inside and out?
is the radiator fan running when it should be?

I have a similiar problem with an overheating Roadmaster. The radiator fan will not come on until the gauge is 3/4 hot. I can unhook the coolant sensor and both fans will come on and the engine stays under 1/2 gauge. I can run the a/c and the gauge stays under 1/2..but turn the a/c off and the thing climbs towards hot. I do not feel the problem to be mechanical/cooling given it runs cool with the a/c on. I do not feel it to be normal as it performed correctly for years. Replacing the coolant sensor did nothing. Something in the ecm or a signal to it would be my guess, Not interested in an expensive "chip". If I can not locate the problem I may convert it to a mechanical fan as found on station wagons/police/heavy duty, Any ideas?