Charging problems on a 88 gmc sierra

Hi, I have a 1988 gmc sierra 1 ton 4 wheel drive with a 350. I'm having problems with the charging system, the amps gage shows between 9-10. We have changed the alternator, starter sylonide and batteries and the problem still isnt fixed. We have a dual battery set up because we have snow plows on the truck. We are hoping to get it fixed soon so we can get back to plowing. Thanks in advanced for any help.