Said dog is house trained but is using bathroom all over my house?

I understand where you're coming from. Dogs that relieve themselves in our homes is distressing to many sitters. First thing I'd suggest is to limit space so the dog can't freely soil the whole house. If you have baby/pet indoor gates, use them to limit the dog to a room that will be easier to clean up messes when not observed. When you're awake, some sitters will keep the dog tethered to them on leash, so they can immediately re-direct any bathroom attempts to a more appropriate location. If you're not already doing so, I'd make sure the dog had plenty of walk time to eliminate outdoors (for me, that's at least 30 minutes walk every 4-5 hours with the exception of overnight might be a bit longer).

I'd also suggest calling Rover so you can discuss with them and determine alternate options that you could discuss with the owner (which may include having an alternate sitter care for the dog with the owner's approval). I would also suggest communicate what is happening to owners when you're ready to do so in factual (non-emotional, non-accusatory language). It may be something like... Rover is having numerous accidents inside the house. I thought it may be due to nerves and being in an unfamiliar environment away from you (which is common) and would lessen, but it has not. Do you have any suggestions?