When I do a quick check, I just test across the battery terminals. One small step better is to go from + battery terminal to a good chassis ground.

Your new post works to confirm my previous guess. Power requirements go up between the square and the cube of vehicle speed. Increasing your speed from 60 to 90 (+50%) means your power to the rear wheels increased by a factor of 2-3, which in turn means your fuel pump had to deliver that much more fuel. The fuel pump has a significant current draw, so I'm pretty sure you have either a tired alternator, or for some reason your alternator drive belt is slipping at higher engine speeds. Check to make sure the belt is not bottoming in either the crank pulley or the alternator pulley. If it is, it's too narrow a belt or an extremely worn one.

The temp and gas gauges are sagging because the system voltage is dropping below the instrument voltage regulated output of about 10 volts.
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In God We Trust. Everyone else, keep your hands where I can see them.