Does anybody know what cities are in big demand for Rover sitters?

Every large city in the USA has a large Rover presence. If you want to move I would pick a city that fits your lifestyle and if you have been a successful sitter your business should continue. Right after the move you should expect it will take some time to get established and acquire new clients.

Depends on services you offer. I have family in Fischers, IN and my daughter can't find a rover host that does boarding availability or even house sitting. Rover sitters are in short supply there. She leaves her dog at a kennel that also provides a lot of play time. That seems to be one city in demand.

I'm in Orlando and it's pretty busy here. Either with people bringing their dogs with them but needed somewhere for them to stay while at theme parks, or people going on vacation. There are a lot of good boarders here, haven't seen as many walkers though.