Don't feel that you had a uniquely bad experience. Nissan salescreatures (along with other brands) lie like rugs. They tell so many Whoppers that it's sometimes hard to separate the lies from the garden variety ignorance.
Anyway, the heat pumps also draw too much power for your idea to work, and they also stop providing noticeable heat at about 24F. You have a couple of options if you want to use a big 12 volt battery to extend your range. I once tested a setup that used a 12 volt storage battery, an 800 watt inverter (square wave works fine for this) and two little 200 watt personal heaters. I mounted the heaters on the driver's seat front frame, aimed forward, and put the rest behind the passenger's seat. By turning on both heaters while driving, I created a little 'warm bubble' around the driver's legs and feet. The cabin didn't get above 45F in 20F weather, but my legs were warm enough to make it tolerable. You could charge the battery externally - just make the capacity large enough - or wire it to the 12 volt system either as a replacement for the accessory battery or to supplement it.
Your other option, as noted, is to just get a large enough LiFePo4 or AGM battery to largely replace the accessory battery for the usual loads. That should be good for a couple of miles. You can get more than that by running the tires at about 40psi instead of the too-low 36psi recommended
Biker Jacket Styles by Nissan.
