What kind of potatoes do I use for potatoes and onions in foil cooked on the grill?

Don't cook em in foil. Brush baking potatoes w butter or bacon fat. S & P and indirect grill for 1 hour w soaked wood chips tossed on the coals.

I am assuming you are slicing the potato and putting butter and onion rounds in between each slice.

I agree, you can use almost any kind of potato; but, I use russets with walla walla onion and unsalted butter.

New potatoes are best. Big baking potatoes must be cut and the texture is not ideal.

You can use anything: russett, Idaho, Yukon gold, etc. Just cook for less time if you use tiny new or fingerling potatoes. Here's an easy recipe:
http://food52.com/recipes/22201-campfire-potatoes