How do they go 30"? That seems really low to me.

Open wheel wells 2010 XL SPECIALIZED Step Deck Extendable Drop Deck For Sale At TruckPaper.com

Yeah, I know you want a lower deck but I can't figure any advantage to an adjustable deck height. Why would you ever adjust the deck height higher? Just get the lowest available and be done with it?

you might need to raise the height if you have open wheels and want to load skids over the wheels, those open wheels sure throw mud and water like crazy, i would want some kind of covers to put over them when possible.

#1 reason is ground clearance, but you need the option to run lower if you need to keep the height legal.

On my DD if i have a top heavy load I will raise the trailer to make it more stable, harder air bags, less rocking.
Sometimes 1 inch can mean a lot of extra miles or a pole car.

It's funny but the guy I exchanged emails with, the one who said, "we can make them as low as 30", was actually from XL Specialized. He passed me to a local dealer and that guy said only Transcraft can probably make such a low trailer. He called the plant they told him, "the lowest deck we can make is 36" off the ground." But judging from the picture link posted by Heavy Duty, it's possible and it was done by XL before.

If I had a trailer like that, I'd run the suspencion at 40" pretty much all the time. But when the load is over 10'4" all the way to 11", I'd change the ride. It'd be great to have some removable non-load bearing cover above the wheel wells. If they make it a 10'1" tandem or a tridem with 72" between axles, there should be plenty of space between the axles to place some pallettes there, when the load requires it.

Maybe you're right ... let them make it 10'1" spread and then build some covers over the wheels (like on your trailer). Of course I wouldnt' be able to load anything over the wheels, but life is not perfect. a 30" high deck could potentially mean 11' tall loads!

You can load some things over wheels that is what dunnage is for.

Call a trailer dealer that knows his trailers temp1

The problem with adjusting air suspension height is in reality they're only designed to be within maybe 1/4" of specs. Anything above or below that is hard on the suspension/bushings/shocks. Maybe I'm wrong, but I haven't heard of any air suspension on a trailer that's actually meant to be adjustable.