You are right to be cautious. The big lead-acid batteries can give out hundreds of amps.

Figure 1. Battery isolator or master switch.

A battery isolator is made for this task. The battery negative lead is disconnected from the battery, the end with the bolt slid onto the post and the original lead-clamp connected to the other end. You can just see in the photo the insulating layer peeping out at the rear terminal. Screwing down the green knob connects both halves together with a large contact area to prevent heating.

Your local auto parts supplier will have them.

You are right to be cautious. The big lead-acid batteries can give out hundreds of amps.

Figure 1. Battery isolator or master switch.

A battery isolator is made for this task. The battery negative lead is disconnected from the battery, the end with the bolt slid onto the post and the original lead-clamp connected to the other end. You can just see in the photo the insulating layer peeping out at the rear terminal. Screwing down the green knob connects both halves together with a large contact area to prevent heating.

Your local auto parts supplier will have them.

Back to the question:

I have a multi-meter, but worry that the amperage may be high enough to blow it even unfused (it says 10A max) if I connect it in series between the aux battery and the incoming + lead.

You can't with the equipment you've got. With the isolator suggested you won't have to.