There is no right answer for this question. The best way to find out is to have a impartial driving instructor ride with you on a regular basis to evaluate your driving abilities. This cannot be a family member or friend.
To keep my pilot license active, I have to have a complete physical on a regular schedule as well as fly with a flight instructor for him to judge how my reflexes and decision making abilities are affected as I age. I am fortunate that I still have uncorrected 20-20 vision and excellent night vision and so far I have not killed a flight instructor. :B
It is impossible and foolish for a person to try to evaluate their own limitations. Maybe you recognize some limitations, but you probably miss other problems that can be deadly.
One thing that is important to remember is that slowing down on the highway, because of your own physical limitations,is NOT being safe. It is usually known as obstructing the normal flow of traffic. There is a huge difference between slowing down to save fuel and slowing down because you are concerned with your own ability to handle your coach when in traffic. Don't use "saving fuel" as an excuse to cover up your own failing skills.
Keep in mind that the rest of the world DOES NOT care how old you are. The rest of the people on the roads will not slow down for you. You MUST be able to interact safely with others on the highway, that are driving their various vehicles at the speed limit, and sometimes above the speed limit, and you have to be able to adjust. Trust me, most people DO NOT care about saving fuel and it is YOUR responsibility to blend in with the flow of traffic and not be an unsafe obstacle on the highway that others have to avoid.
Everyone is different and every vehicle is different. I have seen some very unsafe drivers on the roads of all ages. A 30 year old can be in perfect physical condition but he was not born with the necessary skills to drive a large RV, or in some cases, any vehicle, safely. This is something that comes with experience and even then some people just DO NOT have the ability and mental fortitude to acquire those skills.
Driving a heavy vehicle on the highway is a huge responsibility. It is not to be taken lightly nor is it a Constitutional "right". Having a driver's license is a privilege and just because your license is valid it is still up to you to insure that you are not a danger to yourself, or to others.
Contacting an independent driving instructor to evaluate your limitations is the only safe way to decide. Age has very little to do with your physical skills or decision making abilities.
Let's all be safe out there. :)