Nine volts?! That is serious! I'm surprised the engine still ran at that voltage, viz. the requirements of the ECU and stuff.

What's hard to fathom is that I can't think of what could drain your battery when you're driving it, and wouldn't drain it when you're just sitting and letting it idle. So I'm thinking that it isn't a matter something creating a serious drain on the battery, but rather what is happening to the charging system after having driven the car for a while.

You wrote, if I recall, 20 to 30 miles. Is that about a half-hour or so of driving (highway, or secondary roads)? Have you just tried to let the car idle, stationary, for that same length of time (and with whatever accessories you used when driving such as your headlights), to see if the battery voltage declines similarly?

Sometimes, as the engine compartment warms up, the alternator belts' tensions might change (if it wasn't tight enough) and start slipping (you don't always hear a squeal when it slips). Thus, you've got a good charge when you start up, but it doesn't maintain it later when warm.
Similarly, there are bushings supporting the alternator -- 3 of them, actually, two on the shaft around which the alternator rotates (for adjustment), and another one supporting the end of the alternator's adjustment bracket (near the water pump). These age and go soft, and maybe flex too much when warm, affecting the belt tension.

Also, rpm can affect belt slippage. At idle, the belts might be okay, but when the rpm is higher (when driving the car at speed), they may start to slip and fall behind enough to fail to keep the battery charged.

Another possibility is some defect in the alternator, similarly temperature dependent. You did say that you got a new Bosch alternator -- was it really new, or rebuilt/reconditioned? The voltage regulator is supposed to change the voltage with temperature, but maybe it's cutting the voltage too much. Did you have this problem before you replaced the alternator (i.e., is it the reason you got the replacement), or is this problem new with this new alternator?

Also, any idiot lights glowing (even faintly)? If so, that could be a sign of worn brushes. They may be more troublesome when hot.

Report back.