You do not need to wonder about mileage for larger trucks or campers. With your light weight pickup you are limited to a light weight pop up camper or a small trailer.
We went full time with 2 cats in our truck camper. At first it was difficult to keep the cats from escaping and that happened twice in the first couple of months. After that they adopted the TC as their new home and did not try to leave. Using a pop up would make dealing with cats more difficult. You will need to account for each cat and make sure they are in safe places when you lower the top. You are also going to need to think about ventilation and temperature control for the collapsed camper.
