How often to change the oil?

Sealed up inside the motor. not dirty unless it is ran. should last as long as it would in a can on the shelf.

Don't know about your 2007, but for our 2003 E450 Ford recommends their Motorcraft 5W-20 semi synthetic oil. The filter is the Motorcraft 820S. Both available at Walmart.

I change the oil and filter every 5,000 miles, although I have gone over that number somewhat a few times on long trips.

During long periods between trips (mainly Winter) we take a drive to a mall about 20 miles away to give the engine, drive train, brakes and other moving parts some "exercise". IMO regular exercise is good preventive maintenance.

Conventional dino oil breaks down over time. That means if you only use your RV a few times some years you still need to change oil about once a year at least. With synthetic oil it is possible to go much longer.

I use synthetic oil, and do not worry about how long it sits. I do think that it needs to be started, and let run until it warms up well, to keep fresh oil recirculated from the drain back up into all the engine parts. Without mileage, the age of the oil makes no difference. With synthetic, I plan oil changes closer to 10,000 miles, whenever it is convenient.