I have a Rhino Evo slider and want to know how to set it up so that the timelapse can go up hill or downhill slightly.  I have a MeFoto tripod with a ball head and it seems like I'd have to attach one side of the slider to the top of that ball head somehow so I can tilt the slider, and then put the other side of the slider on the ground (or a rock or something else).  Then I'd have to get another ball head and place the Arc on top of that ball head to level out the arc and camera.  

Does that sound right?  So I'd need to purchase another ball head (since I just have the one)?  Any recommendations on another head to use?  

Thanks!!

Yes. You could do it like that. Instead of a ballhead on the cart, I'd recommend a 1-way leveler, like http://amzn.to/2ncUeLw - this makes it much easier to setup and getting everything straight.