These vehicles don't charge very well at idle simply because the alternator doesn't spin fast enough...most people don't think about the added load from a/c, lights, etc. when they look at the voltmeter.

The situation you describe is normal, but if you want charging at idle (because you spend alot of time at slow speeds) contact an alternator remanufacturer and obtain the smallest drive pulley you can possibly get...this will increase your alternator RPM at idle, and resolve your problem.

Rob in Hell