I bought the 2007 HR Neptune XL used and it's age explains some of the reliability issues. The chassis A/C has required 3 major and expensive repairs in the year I have owned it. The temperature guage always reads at least 3/4 towards the hot side even when the unit has barely run. After running several hours in hot hilly terrain, the generator alarm will go off requiring a pull over to the side of the road and letting the unit idle.
On the good side, it drives well. The steering is much smoother since I added a Roadmaster steering stabilizer. With the 4 slide outs there is a lot of usable room inside, especially for a 36' model. The shower is a nice size. Overall I am satisfied with the unit, I recognize that mechanical issues come along with older motorhomes.
I have no experience with the manufacturer warranty/support.