I wouldn't at all consider those acceptable voltages drops with the engine running (idle or otherwise). A few basic questions:

1) How old is the battery? If over 5 years then it's on borrowed time and may not be able to hold a charge or worse still may not be able to take a charge. After being on a charger, what is the voltage reading across the battery terminals -no loads on? You want to see just over 12V (12.25V-12.5V is what you're looking for in a healthy battery). If you see less than 12 volts then the battery is shot (or your meter isn't accurate). If 3-5 years old, has it ever sat around for an extended period without being charged? A battery shop or almost any half-decent garage can do a load test and will often do it for free.

2) You need to check the output voltage of the regulator. With a minimal load on and at well above high idle (say around 1700-2000 rpm) what volts do you see on the back of the alt (not at the battery) between the B+ terminal (large red wire) and alternator ground (case stud)? You want to see at least 13.75V, preferably closer to 14.2-14.5V. The voltage regulator pack inside the back of the alt doesn't last forever and it's fairly easy to replace. Even if producing the correct volts, anything less than a 1/4" brush extension out of the holder means they're worn and the pack is ready for replacement, and if the slip rings are worn then sooner rather than later. When the regulator pack is out, poke your finger in and check for an excessive brush wear groove in the slip rings.

3) Repeat that measurement across the battery terminals to determine the voltage drop due to resistance of the battery cabling and connections. A 0.1-0.15V drop is normal, while a 0.2 or more voltage difference indicates you've got a cabling/connection problem that needs addressing.

Trying to reduce the lighting loads is no more than a band-aid fix. Your 100 amp alt should be more than enough to handle e-codes plus all the other lighting while also maintaining a positive charge at idle.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now