Trailer Tow Relay #2 is the relay that feeds power to pin 5 of the tow harness when the key is on. This circuit is fused at 30 amps. This is typical fusing for a trailer charging circuit. You would have to have a really large battery on the camper (or your aux battery) that was fully discharged to pull that kind of current.
But this means if you use that circuit to power your aux fuse panel DIRECTLY you would be limited to 30 amps. If you hook that lead to your aux battery(basically hooking the aux battery in parallel with the truck battery), then feed your aux fuse panel through a fuse from that battery, you could pull more than 30 amps without blowing the charge fuse.
However... You now have the problem of two dissimilar batteries in parallel with each other and the charging issues that sometimes produces. I have towed a camper with two Trojan T105 6V golf cart batteries in series for 12V for years and have never had a charging issue. I just make sure to put the batteries on a 6V, three stage charger, after every trip to charge them completely. You could do the same with your aux battery once a month or so.
You would need to be aware of the state of charge of the aux battery at all times. If the battery gets really discharged AND you tried to turn on say all the exterior lights on the aux fuse panel at the same time, you could easily pull more than 30 amps through the charge fuse as the truck battery will now try to make up for the extra current being pulled. Just fuse everything correctly, stay aware of the state of charge on the aux battery and you should be fine. Oh and get some 30 amp fuses just in case. :iconbiggrin: