Jim,
Thanks for the info. Not surprised to hear that the 5.5 just won't go. Phoenix is a fairly narrow coach comparatively, and the trade-offs have to come from somewhere. Seems logical that they would have included it as an option if they could have shoehorned it in.
On a positive note, I've been pleasantly surprised by the performance of the Onan 4K so far, compared to some of what I'd gathered from forum review (remember I had no RV experience prior to this year). I've got about 60 hours on my 4K now, over-nighting at rest stops and Cracker Barrels, and it's been flawless. Punch the button--it starts; punch the button--it stops. Very nominal and agreeable running noise (from my perspective) inside and out. Carries the A/C and all other power needs without a blip. You can hear it load ever so slightly when the A/C kicks on, but the engine still sounds pretty effortless. I've run it 10-hour stretches--believe it could have been 20 or 30 just as easily. Changed the oil at 20 hours per the book, otherwise done nothing but push buttons.
All the best, Mike
