That is one helluva topper! I have a M1101 that has been sitting while I get some inspiration. Still debating if it is too big, but I already own it, is super clean and still has nobs on the tires. I will be wiring it up tomorrow and finally get it registered soon.
I understand your desire to cut the side, but it will devalue it quite a bit. I don't see many M416s with the sides cut out. You can't do slideouts to the rear? In looking at my build, I also am considering keeping the fridge in the truck so it is with me when I do day trips and stays clean.
I plan to build a rack for my RTT and need to figure out a tarp or hard cover for the trailer.