A lot of people add memory foam toppers to their Class B beds - and you can get different thicknesses and densities. I, personally, like a more firm mattress and when I tried the memory foam I felt that I was just sinking into it. Our Roadtrek has the king/twin non-power bench/bed. When made up into a King it is 6 feet long at its longest point - and that is sleeping lengthwise in the van and not across which is also possible. We got a good quilted mattress topper and put that on the cushions and it is very close in feel to our bed at home - firm and it gives support. I find our RT bed with how my wife makes it up to be very comfortable.
As to the length - check out that the bed is actually equal in length on both sides if you are both tall. The RT bed that we have is shorter on one side than the other - one side is six feet but the other side is about two or three inches shorter. Even though I am under 6 feet tall, with distance from the headboard to the end being 6 feet - you don't sleep with your head against the headboard - so I find my feet hit the cabinet at the foot of the bed. At 6'1" in most Class B beds you will need to sleep on a bit of a diagonal - which we have done and it is comfortable enough.
Go and take a look at the Class Bs you are most interested in - and lie down on each bed - only you know what you are comfortable with.