Yes do ask your vet about other painkillers.
And a couple of things you can try are....acupuncture....I hear it can have some remarkable benefits for arthritis sufferers! (yes even canine ones)
Also swimming therapy can help. The reason it can help is while in the water, the body is supported and movement is SO much easier. Regularly done, this exercise helps to keep muscles and ligaments in tone, thus strengthening them, so they will be more supporting to arthritic joints for general mobility.
Hello & welcome....Rimadyl is rather on the low end of quality pain control......There are many newer and better drugs available, now....Tell your Vet you need better pain management with something else....You could also ask to add "Tramadol" to the Rimadyl as a combination therapy....The two together work much better.....
Good for you to have a dog of 18yrs.....Sounds like you have done everything right! I'm impressed......Good luck, Karla