More heat needed in garage. Ideas?

So I tried closing off all the registers and the rear gets more flow but with the small size it just isn't going or doesn't allow enough CFM. The register before this is in kitchen and it is fed by a 12'x 2" or 3" supply. It pumps heat with good temp and velocity. I would like to tee off that but not sure how to the routing under rig. As far as people telling me to camp where there this hook-ups I'm not sure you have ever camped in desert and gone motorcycle riding. Doesn't exist and defeats purpose of having giant toyhauler. I am really trying to not add a another heater as I have little kids. Don't want them burned. Thanks for the help. Keep em coming.