Hello & welcome back.
In most cases, the cause of the fuel gauge jumping around it the fuel level sensor which is mounted on the Fuel Pump assembly in the gas tank. I have the same situation right now in my 2001 Chevy Venture van (basically the same van as yours, but with less options). The best fix here is to have the sensor replaced. I would not really recommend replacing it yourself, unless your are comfortable emptying the fuel tank and then removing it from underneath. I would have it done by the pros. Just one note: there is a fuel level sensor "kit" that can be obtained from almost any GM dealership (Olds, Chevy, Pontiac), and the entire fuel pump assembly does NOT have to be replaced.
As for the windows, I have a trick that I have used several times on my Venture. Get a can of Silicon Spray Lubricant (right next to the WD-40 in most hardware stores), attached the little extension tube that comes with it, and spray the window guides (both front and back part) when the window is down. Give it a good soaking - about 3-4 times up and down the full length of the window. There is some "felt-like" material that dries out over time and make it very hard for the windows to go up smoothly. After the spray is applied, put the windows up and down a few times and I think you will see a much improved function of the windows.
You didn't really ask a specific question about the trip computer. If you have a question about that, just let me know.
Take Care, Greg A.