IMHO, for GMC vehicles that old, 110,000 is NOT low mileage. It sounds like it's ready to give up the ghost shortly.
If you don't drive too many miles on average, then the bad gas mileage might not be a deal breaker and driving it until it dies might be a good option.
If you do a lot of driving and / or having a dependable vehicle is terribly important to you, it might be time to trade up.
Windows not going down could be annoying, but if the AC still works, it really seems kind of irrelevant.
My old car had the AC go up, but has perfectly functioning windows. Good enough.
However, I bought a new car because I commute 50 miles per day and need a dependable car for getting to and from work.
The old girl is still on the road with 185,000 miles on her, but she's the 'backup' car now.