So, you are planning on borrowing a dolly, renting a car and towing it when needed?
Are you dragging the empty dolly to near your destination then renting a car nearby?
2 trains of thought.
1. Rental company will never know you're towing it if you drive away with the car. Liability? Assuming your insurance covers rentals, check with them and it essentially doesn't matter what the rental co policy is, if there's an incident, you're paying whether behind the wheel or behind the moho.
2. You have a brand new Rubi, why not just dolly it if you don't want to flat tow it and save the rest of the hassle?