Regarding the open space under your step/treads - you gonna keep it open ?? I would !
If you don't keep it open i would encourage you to put a door or drawer under each step....anything that would allow someone to put there shoes UNDER the step (or maybe even some frequent items, used during set up etc).
we TRY not to wear our shoes in the coach (still happens) but when we kick them off we don't really have a place to hide them (they get left on the steps to be tripped over or just generally be in the way).
Regarding the steps HEIGHT -
I've never measured mine but I've never had an issue (except on my mother in laws curb)....i just know to stay a few inches away when parking there (to keep the coach tires out of the curb and more "on the street")...and then its only an issue when i let the coach down off the jacks (after I've deflated the bags).
If its too big a step from the ground to the 1st step you can always use a portable/height adjustable step - we have one (and only had to use it once or twice) -
CAMCO 43676 Step
assuming you'll have levelers and won't always have the luxury of parking on level ground...the height to the bottom step wont always be the same anyway.
I've got a jeep wrangler that doesnt have 11 inches of ground clearance and other than some off road trails I've never even come close to contacting any thing under the jeep.....same with my wife's tiny miata.