Our previous truck (LMTV) was about the size of your truck. If I remember correctly, when we repainted the cab from tan to light blue, it used about 3/4 of a gallon. The back was one coat and the rest was two. A few small corner areas had three coats. We painted the inside of the doors and the door jams, but we otherwise did not paint the interior. Painting over red might take more.
As a side note, I have always found white really hard to color match. Are you planning to paint your Total Composites box? Our Total Composites box has several slightly different colors of white. The panels are a slightly creamy-white gel coat, the corner extrusions are a brighter-cooler white. I'm not sure what windows, hatches, and doors you are using, but our Tern Overland hatches, are a slightly grey-ish white. I don't know how much you care how well your white matches. We really like our Total Composites box, and the slight color differences in the white doesn't bother us at all.
It's a little hard to see the shades of white in photos, but:
---First photo is during our box assembly prior to bonding on corner caps.
---Second photo is bonding in one of the Tern access hatches right before scraping away the wet adhesive.