I have 3 former ferals living in a very large converted weather proof dog kennel. Taking them to a shelter is a death sentence. You have to find where it got in, had to be an opening somewhere. Ferals usually rest during the day, becoming more active as night approaches. Best bet is to coax it out with food, early evening or early morning, at the same time for a week. Walk far enough away, wait & watch for where the cat goes into or out of the RV. Start moving the food a little further away each time also. The key is to find the entry while it's eating /out so you can keep the cat out. Ferals will mark their territory but usually never where they sleep, if that's any consolation. You would be surprised how small of a hole one of them can enter, getting out might not be as easy & there could be a possibility it's trapped inside.