That's an oldie but a goodie!
On the broken glass, you are asking about sourcing the gasket material? Something may be different with your 76 model, but if the windows are like the rest of the Sunny's or most any brand camper, the outer vinyl gasket you see is more of a good looks fill up the gap gasket. The glass seal to the metal frame is a glue bonding the glass to the metal frame creating a positive seal.
Unless you have a mobile glass repair place to come to you or you can tow the camper to the glass repair place, they deal with the glass replacement and bond in new glass. That or you remove the entire window frame and all and take it too them.
Have a closer look, you can most times see the bonded glass to the metal frame from the inside of the camper.
Hope this helps and please report back what you find. What I stated above fits Sunlines I know for sure from later 80's all the way to the 2007's. Just not totally sure on the mid 70's ones.
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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