Yearly is a good number - change it at the end of the camping season; you'll be good as new again.
If you exceed 5K in a year but not by much, no big deal - modern non-synthetic oils hold up well up to 7500 miles. If you run full synthetic, you can run up to 15K miles.
The reasoning behind changing it yearly is to displace moisture.
There's always so much hype and anecdotal stories about the oil change duration - most of it exists to sell you product. If the oil is clear or has a little carmel color at 3K, don't change it. There's a lot of life left in it.
As n7bsn stated - look at your owners manual. It is the best guide.
If you don't have the paper guides that came with your unit,
Ford has PDF versionsand they say 5000 miles (in my 2000 year manual) under normal operating conditions. Yours may vary - look it up.