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You need it as close to 28 as you can get it. The range is 27.5 to 28.5.

Is that at idle or 2500 to 3000 RPM?

At 3000 RPM you should get 27.5 to 28.5 Volts.

How can I tell what the RPMs are without a tachometer?
And that reading is when I first start up the vehicle. I'm out side with the engine bay open. It runs a little higher when it is first warming up, but I don't really have to play with the choke or the throttle, it fires right up and stays that way.

Get a tach or accelerate the engine by hand until it sounds like it is running as fast as it does when you are rolling down the highway at 50 MPH in third without OD. Or if you got a lot of faith in the engine pop the throttle wide open. Then once you can clearly hear what 4000 RPM sounds like reduce the power 1/3 rd and read the voltmeter.

I ordered a digital tach. They are like $12. I don't trust my hearing, ever since I had brain cancer, I have ringing in the ears.

I bought a 24 volt charger, and I'm keeping both batteries fully charged. Both batteries had been drained before, and I might have unevenly charged them. I also have a line running off one for my 12 volt GPS. Is that going to cause a problem?

Tapping one battery for 12 volts always sets up an imbalance. This is solved by switching the battery you are tapping for 12 volts on a regular schedule.