As far as your intitial inquiry, 30ft Trailers are not big money makers in the HotShot industry. If you could find a local manufacturer that produces soemthing like "AC" units, and run their stuff exclusively, a 30 ftr might make it. But, those types of customers are few and far between cause the guys that get those customers just don't give 'em up, period.
Biggest moneymakers in the HS industry are 40 -53 ft Lowboys. When freight is good, or abundant, it's nothing for these guys to be loaded at capacity with multiple customers shipments on their trailers bringing in $4-$5-$6 a mile. But, just as the Expediters have been experiencing these last couple of years, freight is slow all around due to the economy and higher fuel prices. Still, there guys out there running good bringing in $5K a week easily. (in the past, IOO's with 1 tons and 50ft lowboys could make $5K in 3 days if they were dispatched right or had a good customer base)
As far as single axle VS a dually. ALWAYS go with a dually. As mentioned above, they are safer and offer more stability when pulling trailers that might have 15K - 30K lbs on it.
I have always ran with Duallys. I will never never pull some of the stuff I've pulled with a SA no matter what. Even with a Dually, there have been times when I had like a 36-38ft - 16K lb 5th wheel behind me running 60-65 mph down the road when an 80K lb idiot with 18 wheels came flying by me running 85 mph. When that force pushes you onto the shoulder, and when your butt puckers so tight getting that trailer back into the right lane, you'll thank the Gawd above that you were in a "Dually" and not a SA, know what I mean. (maybe not, but you might sooner or later)
Sent you a PM, OK.