How to attach a TV mount to a resilient channel wall?
Your best option is to cut a peice of 1/2 " finish plywood around 36 inches wide and tall enough to split the resilient channel at the top and bottom. That way the drywall will be supported by the channel as will the plywood. Carefully cut the drywall to the same size and firmly secure the ply through the channel into the wood without crushing the channel. You can fill the seam and sand it but choose a filler that remains flexible because wood and gypsum expand and contract with heat and moisture at different rates. Now if you so choose, you can prime and paint.  This method will allow you to support several hundred pounds, assuming you wall can. My TV is a 100lb 70" with a 30" long mount which allows me to turn the TV beyond 90 degrees. In my application I added plywood behind the drywall and replaced the drywall. As you can see from the image I didn't fill sand android paint but that's an optional step.