You may already know this, If you are boondocking in a camp ground, (no hookups) you can't run a generator at night anyway, which is usually when you need the AC.
For about $300.00 you can buy a new generator that will run the AC, in the day time, but they are rather noisy and occasionally you will get a camper that complains about the noise.

i know on later 70's models its above rear window,i had the measurements some where.Hi-Lo purposely boxed an area in this spot above that window specifically for the purpose of installing an air conditioner.I think Hi-Lo started using the steel tube construction in 69.i have a 78 so i know it continued till then & 79 too.if your has a wood frame this info may not pertain to your Hi-Lo.i do have pix of the frames used but don't know how to post them here,sorry.Good Luck