1990 Chevrolet Van Charging while driving

I have a g30 van, 1990. The engine runs like a swiss watch. When I fully charge my batteri, the car starts easily, and the voltagemeter for charging shows around 13, give or take 1, it's not an exact scale. When the car has started, I can let it stay with the engine running, and the voltagemeter still shows 13, however much electrisity I put on (heat van in the front and in the back, cabin lights, stereo, head lights). Everything looks just fine. When I start to drive, it will seem perfectly ok when i'm driving in towns og other places where I don't drive fast. But as soon as I get on the highway, the voltagemeter starts to drop, and after a short while it has dropped to below 8 (which is the lower limit of the voltagemeter). Then I loose the electrisity for vans/music/lights. And if I let it go too far down, the dashboard is acting curiosly; blinking, beeping and the speedometer indicates that my speed is 140 km/h, however slow I drive. But - if I turn of all electrisity thieves, and park by the side of the road for five minutes - with the engine running, it will pretty soon lift the indicator of the voltage meter back to around 13. But if I switch on the head lights - it will start dropping againg. And when I get on the highway again (with no headlights), it will drop to below 8 again pretty fast.

It belongs to the story that this car should have two batteries, but there is only installed one. I will reinstall a second battery, but I would like to know if this problem has something to do with the charging, cause I can't imagine that the amount of batteries can inflict so strongly on the charging?

When I have sorted this out I will buy two new batteries, but I'm afraid of damaging two new batteries if this problem will continue after I have changed them.