I use the kind with a stone crock and have heard that they differ from the other kind with a metal pan that sits on a heating element. I know that my roasts turn out differently than my older sister's & she uses the other kind.
That being said, I used to just use a bottom round roast or a bolar roast and pour a bit of Kitchen Bouquet or Gravy Master over it & then sprinkle with either an envelope of dry onion soup mix or granulated beef bouillon. I also put carrots in the bottom, then the roast & then potatoes because that's the order given for my pot by the manufacturer. Oh - and I'd add a cup of cold water (before the KB or GM) if the roast was going in frozen...again, per manufacturer.
In the past couple years however, we all decided we were a bit tired of it that way. I still use a bottom round roast, but I've been cooking it longer and then shredding it. We either eat it over mashed potatoes or as a sandwich on a hard roll with horseradish sauce. Yum!
So now I season it differently and would probably use the same things if I were making a regular "roast" as well. Now I always use water in it as I really want lots of broth.
2 c water
3 tsp granulated beef bouillon
3 tsp garlic powder
1 Tbl oregano
1 tsp coriander
1 tsp chili powder
½ tsp cardamom
1 tsp Worcestershire sauce
a few drops of Liquid Smoke
2 Tbl seasoning salt (I use a "no salt" version)