battery getting drained while driving

All alternators put out less juice at idle speeds; it is not a matter of shutting off. So that behavior seems normal, and with a heavy 48 A fan load, it will no doubt be discharging the battery when at low idle.

You need to do the test as suggested with meter on the seat and watch what the voltage is with other loads, and then when driving to see if the belt tension is adequate. The alternator pulley can be slipping and not make any noise......How are you setting belt tension with the manual set idler? Keep in mind that you are pulling a full HP to run just those fans so the alternator pulley is very heavily loaded.

The other possible problem is that the alternator has 1 or 2 of the 3 diodes sets blown out. This would cause your problem but you could only measure 14.7 volts at fast idle if the heavy fan load is not on.

Are the fans on a relay with temperature control? If not, it would be a good idea the change the fans to run only when a thermostatic switch activates them. There is likely little need for them at cruise speed.

A poor ground will not cause the battery to undercharge or discharge per se (but then you likely would have serious starting problems even with a good battery charge). But an erratic ground could cause the alternator voltage to spike and damage the alternator. So check all grounds to be sure that they are to totally clean metal.

Have you checked your voltmeter for accuracy? Get a brand new 9V battery; it should measure 9.6 to 9.7 volts.