Well, as far as I know in terms of electricity consumption (in case you’re running it off an inverter), pressure cookers use less electricity in total than a slow cooker to cook the same dish. Though I’ve not been able to find any definitive proof of that, and for your setup, I won’t recommend you to use one - as Van_Dweller already explained why it’s not a great idea. It won’t work unless you decide to get a backup generator.
Still almost all of the people I know that live/camp out of a van prefer to cook outside and their equipment tend to be outdoor stoves that are either propane or white gas. I believe it’d be a better solution in your case.