Thanks to all the posters on this thread who have inspired me to come up with what I think is a great solution to the ridiculous cup holder Thor has put into some of their coaches.
I found these
sewer/drain hub pvc couplers at the hardware. They are thinner than regular schedule 40 PVC fittings. The outside diameter is 3 1/2" and inside 3 1/4". They fit very snugly in the forward holders. I wiped them down with acetone to break the glaze and painted them satin black with Krylon Fusion spray paint.
Unfortunately, in the rear cup holders they do not fit snugly and will flop around when a water bottle is put in them. So I tried to find a way to make them snug in the real holders. I found these aluminum sprinkler pipe gaskets that work perfectly.
They are a 3" "Wade Rain" gasket and can be found at a commercial agricultural irrigation dealer or ordered
here. A little dish soap for lube and then stretch and slide over the couplers about half way down.
So now I put the couplers in the forward holders with the gasket on top.
And flip them over to use them in the rear holders where they now fit very snugly and will securely hold a full water bottle without falling over.
PM me if you have a specific questions.