Verynice, I've spent a few hours staring at the framing on mine so here's my thoughts:
The ceiling joists are 1x2 in my '75 (using "joist" and "1x2" together makes me chuckle). I would not mount anything to them; I think the roof would just sag and puddle. Not to mention holes in roof bad...
The idea is to transfer the weight of the rack as directly as possible to the lifting rams without distorting the shape of the shell. If you were to mount a rack to the roof I'd do it above the bulkhead at the entrance to the CO but still I would bolt to the back panel; it's much stronger than those 1x2s. On mine the curve of the aluminum sheet keeps the roof from even touching all the joists so brackets bolted through the roof would dent it.
If you mount the rack to a vertical surface through bolts as opposed to lag screws will give the benefit of much higher compression between the rack and the siding, increasing the friction between them and adding to the shear strength of the joint.
I have no clue why Alaskan mounted the front of the rack next to the window instead of further forward where there's more support but it seems to work.
Anyway, hope this helps your thoughts. Bruce