Here are the sizes:
http://wiki.answers.com/Q/What_are_t..._king_mattress
I believe the king in our ES is 72X80
Absolutely, absolutely: Place a Select Comfort (Sleep Number) on a solid foundation (plywood, etc) We used our Queen in our other 5th Wheel and it was great (plywood base). Then we put it on a regular bed foundation and I never got it comfortable...Sounds weird, but true. When we ordered our King model for the house, the people at the store said the same thing.
We have been comfortable on the regular (RV) King mattress in our ES. So, changing it over to a Sleep Number is not a priority right now.