Tough question, Phil, and one a number of us have faced. The differences tend to be hidden in the details and even when enumerated they usually don't sound like they are worth the hefty premium. I also suspect that high end models are priced somewhat higher simply becasue people shopping in that tier are more focused on quality & amenities than price.
I can't speak to those particualr brands, but here are some typical differences to look for, that is if you are able to dig out the infor.
o Stronger, stiffer frame. May be thicker steel or higher tensile strength steel, welded vs bolted joints, more/better cross bracing
o More standard features (just compare the standard vs optional lists)
o Higher quality cabinetry - better grades of wood and better construction. Look for solid woods thrughout (can be difficult to tell these days) and the depth of the shine. High quality woods just seem to glow with a rich depth whereas lesser grads just have a shine.
o Grade of carpet and carpet pad (difficult to tell unless you are more familar with carpets than most). Many RVs have really cheap thin carpet pads and only average carpets. Good carpeting is quite a bit more expensive.
o Grade of uphlostery, both "leather" and fabrics (again, hard for most people to judge)
o Amount and type of insulation, especially in floor & ceiling, slide rooms, etc.
What you generally won't find is many notable differences in the systems and appliances - the water heater, pumps, a/c, washer/dryer, fridges, etc. will be pretty much the same across the board.
Take a look at the Keystone Everest, which is Keystone's luxury model and therefore perhaps easier to compare to the Montana in terms of construction, amenties and price.
