Slow cooker meat recipe
I'm not sure the altitude is a concern when cooking Swiss steak. You're essentially doing most of the cooking when you sear it on the stove top. You finish the cooking by raising the internal temperature gradually in a Crock Pot. The reason for using a Crock Pot as opposed to the oven is because the lower temperature setting slows down the rate at which the internal temperature of the meat increases and allows time for flavors to fuse and develop in the meat, and to some extent tenderize it a little. 

If tenderizing the meat is the concern, the longer you cook it, the more tender it becomes. But longer cooking does not necessarily imply better tasting, as the flavors can become too pronounced and the meat itself can dry out. But this is in extreme cases and any negative effect to the taste will be negligible if it happens to be cooked an hour or even two hours longer than necessary to bring up the internal temperature to USDA recommended temperature of 160F. 

If you have a recipe that gives you a specific amount of time to cook the meat but does not mention adjustments for higher altitudes, a fail-safe method would be to cook for time x 1.5. So if the recipe says to cook it for 2 hours, cook it for 3 hours. If you're in a hurry, just sear it on the stove for a few minutes each side to lock in the moisture and flavor and then throw it in the oven with some vegetables in a pot, cover it, and let cook for 20 more minutes for every pound of meat at 400F and bon appetit!