We have one for my MIL. hers is small, (she only weighs about 110) and we have a small carrier that can go on the back of my Truck or Buick.
Some ideas here.
carriersAs to size, remember the batteries are the limiting factor, if you are looking at using it with an RV, ensure you have a good means of recharging while travelling.
9 miles is a long way, but chances are thats with new batteries, a 50 pound child aboard, and all downhill. Look for specifics, and ensure battery size is commonly available so spares or replacements can be easily found.