We bought our 1998 Montana in 2001 with 71,000km. It was reliable, and I was surprised at the gas mileage. The leveling pump was a nightmare, not shutting off and causing the van to bounce violently many times; neither the dealership nor GM cold help us. I had a garage disconnect it, after we yelled at the dealership service manager for their ineffectiveness. I am still burned up about this.
It was in 2005 that the power sliding door quit and the LED on the clock went out.
In 2006, the air conditioner quit on a family vacation while we were stuck on a hot freeway.
Then the head gasket problem reared its ugly head in 2007, the digital performance readouts began fading, and the van took about a year and a half to die, with ever increasing engine temperatures; finally being driven to the junkyard at 219,000km, when the temperature would go from stone winter cold to above the red in five minutes flat.
None of the problems cited were repaired because of high cost.
I have fond memories of the van because it was the vehicle of our young family years and big vacations, but it essentially was a lemon.
Sounds like you needed a good knowledgeable mechanic.
Tip, stay away from the dealerships when the vehicle is off warranty. The mechanics do not want to work or see the older stuff, and will give you a high price to repair so you will go away.
Everything on your list, the total of EVERYTHING could have be repaired for $2000 tops.
Sounds to me like your van was a great van, and could have been kept in service with PROPER KNOWLEDGEABLE service.
Please find a knowledgeable mechanic.
First, your air shock leveling system was just the leveling switch located above the rear axle ($35). The tip was when you said almost bounced off the road.
Also, head gaskets on your van should not be over $800 and with a warranty. I give a 80.000 mile warranty. Your other problems are minor things, but become huge when repair prices are over inflated.
Sounds like your van was a great van, but because of mechanics who did not know or had time to fix your van at a reasonable price, it became unreliable... too bad.