1994 Ford F-150 start/battery and idle issues?

I have a 94 F-150 with the 300 straight six. With the key on, the battery will read low. When I start the truck, the battery gauge will read as it should. In some rare cases, most usually when taking off with the headlights on, the belt will scream like Hell and the gauge drops extremely. Usually if I turn the headlights off and drive it for a second the belt will quit and it'll return to normal. If the truck is cold when I first start it, and I try to turn the heat up, it will bog the engine down and kill it. Has a noticeable misfire at idle. In even rarer cases, and it does it more often when under an electrical load, the battery gauge and idle will drop and raise repetitively. I was personally thinking it's a MAP sensor or the IAC. Very occasionally, while driving, I'll hear a whistle type sound coming from what seems to be the engine, which leads to me to believe it may also be a vacuum leak. Any response would be greatly appreciated.

MAP sensors and IACs have absolutely nothing to do with the battery. The scream when you take off with headlights on means the alternator belt is loose. Turning the heat can only affect the engine if you have a bad vacuum leak in the heater controls. Even then, it shouldn't hurt unless you have other vacuum leaks under the hood.

Fix the vacuum leaks. Nothing will work right until you do that. Make sure the belt is tight and in good shape.

Putting a load on the alternator ( turning lights on and other electrical things) will cause the alternator to turn harder, thus the belt that turns it, might tend to "slip" causing a squealing noise. Most likely, the belt needs to be tightened up a bit, or if badly worn, it should be replaced. The idle on a cold vehicle should be HIGH when first started, because it will be getting more gas then normal. ( Cold gas does not burn good, so the computer ADDS more gas to keep the engine running and it also speeds up the idle until the engine warms up). IF the engines does not speed up idle, it could be a sensor not working, possibly the Coolant sensor which senses how cold the engine coolant is, and tells the computer to add more gas, until the sensor senses the temperature as being where it should be. The coolant sensor tells the computer to also stop using other sensors, such as the Oxygen sensor, and so this will not be working until the temperature is okay. BUT again, if the coolant sensor is bad, then the computer does NOT see the engine as cold, and keeps the idle low, and the fuel low, and the oxygen sensor is working and THAT then senses that the mixture is lean, which could then tell the computer to RICHEN it, which the computer does. Then it speeds the engine up. The Oxygen senser then senses the mixture is TOO rich, and tells the computer to back down, and so the engine slows down. This is repeated over and over, Up and down, cycling. The IAC could possibly be bad, but so could the MAP, all which cause the engine to run badly. The whistling noise could be a vacuum leak, which WOULD affect the MAP sensor, causing the engine to also run bad.
The best thing to do, is to take it to someone who can check EACH sensor and make sure they are all functioning as they should. A car that is 22 years old, can be a candidate for many sensors going bad.

Have the Alternator belt adjusted, or if it is a one piece serpentine belt, have it checked and adjusted.