The idea is for the portable to first see the batts in the 12s instead of in the 13s. Once the portable starts, then apply the 7300.
If you switch off the batteries and then switch back on that will do that ok. You do have a few moments with no 12v at all in the rig when the switch is open and the gen is not running the 7300 yet. I have no problems with that in our rig doing that except the Trimetric thinks the world has come to an end and you have to start over with that later.
It gets complicated if your gen is built in with a transfer switch, so you have no control what it runs as soon as it is on. Also, you need the house batts to start the gen!
Just have to figure what works with your particular set-up.
