National Caster (aka Service Caster Corp) has 12" pneumatic casters, 3.5" wide with 450 lbs. rating. They also have a 3" wide semi-pneumatic with a 850 lbs. rating. They offered good advice over the phone when I was trying to choose casters for my dolly
They have a 4 position lock option (every 90°) for their full swivel casters . Locking the front two, while leaving the two near the tow bar free, makes maneuvering much more controlled. I lock all the casters on the dolly to push it sideways. You can see an example of their position locks on the casters I chose in the photo below.

You can see the
CURT Adjustable Tow Bar (#19745)
that I added to make manuvering it more controlled. My dolly was made to handle my 4,200 lb camper so it is a little bigger than what you need (scabbed 2x10; 2,000 lbs. capacity casters).
You can also see the Sea Dog 1/2" eye nuts I got from
Fisheries Supply, a marine supply store. These are in each corner on the sides to make pulling the camper side ways easier. They were significantly less than an extra pair of Curt tow bar brackets.
