My sister and BIL have a GMC. (I am jealous, but I am looking too.)
The first thing you should do is get into the GMC groups. The best one is at GMCforum: Welcome to the forum
This is where the people in the know hang out. There is decades (well, 4+) and millions of miles of experience there.
There are also several groups on FaceBook.
There is amazing parts and technical support available too.
Now, about curtains...
I was helping Mary (sister) while (BIL) Matt was stuck with other things, and yes, to deal with the curtains right (as in replacing them completely) the cabinets have to come down. Fortunately, this is not hard to do with two people. Unplug the wires to lights, remove the screws. Going back up? Just have the lifting person hold it in place and run the screws back in.
When you get on the GMC sites, find a local rally to go to so you can meet some other owners. I am hunting for a 23 and have not had a lot of luck so far.