[burnin oil]

It sounds ds like the new wire feeding the relay is a stand alone circuit that is independent of the truck circuit other than being fed from the alternator. If this is the case the the addition feeds its on circuits or fuse panel then the original wiring is still intact and the amp gauge should read the charge level going to the truck wiring provided it doesn't go outside the gauges calibration area. I believe the key is that ots two circuits with the amp gauge on one and not a parallel circuit that can feed around the gauge.

Edit: I just reread his intended routing and Accelo is correct. If the OP leaves out the jumper to the original voltage regulator plug what I typed should be what happens.