The 500 mile limit without starting is for the transmission, not the battery. We had the same problem once on our Lincoln MKX, the key didn't get turned far enough back and the ignition was still in the ACC position, and we arrived with a dead battery. Now I double check that all the dash goes dark, so far in 10K miles no other problems.
Just put the transmission in neutral and turn the key as far back towards off and that should fix you up,unless you have the push button start/stop, then you have to pull a fuse or get a charge line.
There is also a device that will disconnect your power when you plug in the toad, can't remember the name of it.
Dennis & Debbie Z (Retired) F415847
Pipe Creek, TX
2008 HR Endeavor 40PDQ / 2014 Grand Cherokee Toad