You might consider trying a Ruffwear Webmaster harness. We had one for Murphy, our three-legged Lab/Chessie cross. He used it all the time for going up the ramp into the trailer and the pickup. It was invaluable during his amputation and subsequent total hip replacement. It is lightweight enough that it can be worn for long periods of time. We used to laugh at the harness turning him into a "piece of luggage" when he decided to go somewhere we didn't want him to go.
The photo was taken at Cape Disappointment WA in the summer of 2013, which was six months after the THR.
PS. I am in my 70s and had no problem helping him load up with this harness.