I am curious about using an electric blanket (in the cold season) plugged into an inverter on how much drain it would be on the battery?
It would easily be enough to drain a cranking battery overnight, especially a conventional electric blanket for the home. They have electric blankets at truck stops that use less electricity, but of course they take a lot longer to warm up. But, when they get warm, they're warm.
Anything that is a high amp draw or a constant draw should be run off an auxiliary battery and not the cranking battery. A cranking battery is designed to provide lots of amps for mere seconds, and are not designed to provide trickle amps for something like an electric blanket. The lead plates simply aren't thick enough for that.
But here's the problem, when it's colder, the battery has less amps available, and the colder it gets, the more amps an electric blanket will draw, so what little amps the battery has gets depleted even faster. You wake up in the morning and there's simply not enough cold cranking amps for the high amp punch needed to start the vehicle.
A cranking battery has one job, to crank the engine. Don't try and use a cranking battery for something it's not designed to do. At the very least, a marine "deep cycle" battery should be used for inverter usage. You need a separate battery, preferably mounted inside the van where it's warmer, and isolated from the cranking battery, from which to run an inverter. Yeah, people run all kinds of stuff on an inverter from the cranking battery. These are the same people who curse the POS battery for its premature death.