IF you can tow 4-down, forget the dolly. 4-down is easier and cheaper. What you need is a baseplate and towbar. More than one company makes them...I use BlueOx for my baseplate and Roadmaster for my towbar. Then you have a choice of using magnetic lights or a diode kit for your toads' brake and marker lights.
I would suggest you not doing it yourself...at least this time. Check with an RV dealer or Camping World. Most likely under $1500 for everything.
IMO with your motorhome and the weight of the Ford Fusion, one of those "aux. braking systems" are not needed at all. But that's MY opinion.