[writchie]

You could configure this so that when the trailer is attached your "castering" wheel is just off the ground. But I can't think of any good reason why it needs to be castered or powered. Your design cg should be close to the pair of driven wheels. You will probably need another undriven wheel forward that also remains off the ground under normal conditions.

when you hitch the trailor, it's tongue weight will move your CG aft. The hitch will add to your stability. Nearly all of the normal force from your robot mass will be on the wheels and some from the trailer as well.