If the new batteries weren't fully charged when installed the alternator is bringing them up to full charge. I wouldn't expect that to last more than the first day though.
Amps is the draw from your alternator. If the draw is very low it will be close to zero on the plus side. When it's above zero on the plus side it is charging the batteries, like after you start your truck. If its on the negative side your alternator is not charging.