My f150 sat for a year and I recently started driving it again. Everything works and the motor runs good for having 235,000 miles. Tonight as I was driving the amp gauge started dropping and the tirn signals would not blink. Could it be the Alt?

 

YES. Or it could be a bunch of other things.
Bad battery, bad ground, bad connections on the alternator. Dirty connections on the battery or a bad guage. My first guess would be to suspect the battery shorting internally from sitting. They just don't make batteries like they used too.
BUT:
Test the alt output with a volt meter. Test it at the battery. Volts should be close to the same/same. Your hoping to see 13.5 volts with the engine running or a bit more. You can use just one side of a good set of battery booster cables. Put one on the battery GROUND post and the other clamp to a good ground. Like the truck frame. Any change? If it gets better check all grounds and cables. Not sure what engine you have but on my 300/6 the positive cable off the alt is very close to the exhuast manifold. This heat kills both the cable and the plastic connector on the back of the alt. I covered my cable with aluminum foil to help shield it from heat. Several trucks have had alternator/engine fires and I believe this is part of the problem. Check the post on the guy who wants to wire a new volt meter. Check my reply on the volt meter for the cigarette lighter.

If you are not able to check this stuff yourself go to Autozone. They have a complete charging system diagnostic system that should tell you whats wrong. The test is free but they expect you to buy repair parts if needed. If you pay attention to what they do it's kinda like getting a free class in automobile electronic diagnostics/repair. All in all, not a bad trade...