Rear vertical stud spacing distance?

We used to have a smaller Catalina which had wood framing with batt insulation, so I assume yours does too. You can push on the inside walls and try and find where the 1/8" luan plywood doesn't flex, then look for the tiny staples holding the luan in place. You should be able to find one vertical stud by looking for the tape they use to cover the side-side joint in 4' wide sections of luan ply. I don't think you will find studs on equal centers. IIRC, ours varied from around 14-16". I couldn't get the stud finder I had to work on our Catalina, but they're not all the same.