I got one of those trucker's 12 volt electric blankets for my daughter's
Christmas present. (OK, a true "go Dad" kind of present.)  Trying it out, we
found that even on full, it only gets kind of warm--nothing like the ones
you plug into the wall. After you use it, you can't tell if you warmed it up
or it warmed you up. Is ours defective? Dunno. Little like putting a cold
6-pack into a Dometic: what's cooling what?

BTW, the blanket has a shutoff for 15 or 30 minutes, so you can't leave it
running that long without resetting. Very little electrical resistance
IIRC--maybe that's why it doesn't get hot.

The 12 volt compressors work great but will overheat if you keep them on
long enough to inflate a nine foot diameter swimming pool. That's like, all day.