I believe your VET can give you better answers than most of us here on RV.net. There is one member that is a vet and maybe he'll chime in. But asking for a diagnosis on a forum is like playing russian roulette with your dog. Get him back to a vet pronto!
Example: My dog started peeing in the coach. Something she has never done. Suspecting a urinary infection, I took her to the vet. Turns out she is diabetic and just started showing symptoms. We caught it early enough to keep it under control. Now, three years down the road and she's still doing well...
Your pet is small and old. Harder to fight off diseases. As we travel different places they're exposed to many different types of "stuff". He could have picked something up off the grass..get him to a vet.