For about the past 4 months the hotwater in the upstairs shower/tub has been getting worse. Cold water is GREAT.. but hot is not. Changed shower heads.. and that didn't do a thing... Checked all other sinks in house.. and hot water pressure seems to be fine there.
I need help... I have a 72 gallon gas heater... someone said to drain it, and see if its filled with junk?
Any and all suggestions will be great!

You also might want to insulate the pipes.

Mineral build-up in the water heater can get vinti the system and clog the inlet ports of a faucet. That's what I think has happened here. Shut the water off to the tub and open up the valve. Check and clean the inlet ports on the cartridge. Now turn the water back on to thr open valve to flush out the supplies. You should have a full stream coming out of the open valve. You FLUSH not DRAIN your heater.
For long life and fewer troubles you should keep your heater clear of mineral build-up by flushing on a regular schedule. Let me show you how. Attach a hose to the boiler drain at the bottom of the tank. With the pressure on, open the boiler drain and let it run until the water runs clear. You will see a spurt of red,(rust) followed by white grains,(lime or calcium carbonate). This shouldn't take more then a few minutes. Do this monthly to keep it clear. Don't forget to flush it out every month. Your heater will thank you for it. Hope this helps, Tom