Not sure how to fairly charge 2 overnights. What do you do?

Since it sounds like this is outside of your coverage area, it seems reasonable that you might explain that you'd love to care for the dogs, but would need to add a cost adjustment for the additional mileage.

For house sitting - I stay with the dog & they get the same level of care as when I provide boarding ( inc. 4 walks), and it disrupts one's routine to pack everything you'll need to be away from home, so I'd suggest house sitting visit rate should be at least the walk rate times either 3 or 4. So, yes, I'd agree that $30 (or even $37 w/2nd dog seems too low when you charge $15 for a visit/walk).

30 miles is a really long way. It prohibits you from taking other work and it's an inconvenience for you to pack up and move. Personally I would decline the request because of the mileage. Or double my rate because of the distance plus the 2 dogs.

Would it be worth your time if you charged a "daycare" rate on top of the two overnight rates since you have to stay at the house until the afternoon on your last day?