I'm new with any vehicles. This is a 1991 Mighty Max problem.

This only started today when driving around:
I don't actually have a tachometer but it seems when I'm stopped, idling or going slow (low RPM), the Brake Light, Battery Light, and Maint Req'd light are lit, and a slightly dimmer than full brightness.

As I speed up or go higher in RPM, the lights fade until the RPM are high enough that they just don't show anymore.

It seems like if I'm driving for a while with the stereo off, the lights will disappear no matter what the RPM. If driving for a while with the stereo on, after that, they come back on. (This is only very mildly tested so don't take this into consideration fully unless you know exactly whats wrong)

Oh, another thing that might help: I've been low on gas all day, but I've never seen these lights when I'm low on gas.

Any clue as to what's going on with my truck?

a little confused about yur situation , but , at many times when the engien is running at a really low rpm , the ECM registers a low ignition signal frum the distributor and things the engine is "cranking" or not running and emits the cluster lights , like when they are on when yu crank yur vehicle and then they go off. There could be a service issue or idle issue.

have you tried tightening your alternator belt? I had water pump go bad, which caused slack on that belt and all of the lights came on as you described. That or time for a new alternator.

Probably the voltage regulator inside your alternator is bad.