I've had a GD Momentum 388m since April. I've put about 5000 miles on it and spent about 2 months living in it. In the past I've had a class A and a Forest River 5th wheel.
Overall, I'm very happy with it. I haven't taken it in for any service. I've had a few problems that I was able to fix myself. Some trim came loose, the awning switch was wired backwards, the fuel gauge was wired backwards. The owners forum will give you plenty of help (granddesignowners.com). I haven't used GD support, but all reports are it is very good.
Hope this helps.