Sainsbury's mulled wine is gorgeous. I've given up making it, the ready made stuff is far nicer than any home made I've ever tried.
I think if I was going to make it in a slow cooker I would heat it through on the stove before putting in the slow cooker to keep warm. It would take hours to get up to temperature from cold in a SC and I think most of the alcohol would evaporate off.
The Schwartz Mulled wine bags are good - buy red wine from Lidl or Aldi, add sugar to taste and orange and lemon slices for authenticity. You must add some sugar tho, revolting without and I don't take sugar in any other hot drinks