We had a 2007 Mandalay Valencia 38C. It was a 38' with the 300 ISB. It was a very nice entry level, budget MH. However, you are correct that it was under powered. It was fine going down the road on flat land, but it struggled in any mountains, and I'm talking east, not west. Did we make it? Yes, but not without some stressful moments for us. We never took it to the western mountains. We towed a CRV, which is about 3,500 lbs.
Our current coach is 40' with the 400 ISL. DH says it's a night and day difference driving, even when trying to maneuver between traffic situations. Plus, you'll need to watch the weight limits. It was just the 2 of us, and we were really close to the weight max for a 2 month trip.
I agree that floor plan is supreme, but our Mandalay had what I call a "standard" MH floor plan. Our current coach is very similar except we now have 2 bathroom sinks, and the fridge is a double door and is on the driver's side vs. the passenger side. I've seen so many of these similar floor plans, that you should be able to find another
that the DW likes.
If it were me, I'd keep looking. There's a reason the price is so good. As stated. it's a very nice entry level, budget RV. There have to be less expensive/lighter things in it to be able to have the lesser price point (such as the smaller engine).
Good Luck!
