For my 2010 (actually a 2009 Ford chassis) the manual says to change the oil and filter every 7500 miles or 6 months for normal conditions. For special conditions it says every 5000 miles, 4 months, or 200 engine operating hours.
My driving is mostly on the highway under normal conditions, but I do tow the Jeep. Since Ford says these are the maximum limits, I actually change the oil more frequently.
You may want to do the first oil and filter change sooner. Some people do the first change at 1000 or 2000 miles, although the manufacturers don't require it. The logic is that there can be bits of metal and sand casting remaining from when the engine was manufactured.
If you decide to do your own oil changes, Wal-Mart has the Ford oil and filter. Also consider purchasing a Fumoto Valve to replace the drain plug. They are excellent quality, and the ones with the hose nipple allow you to drain the used oil straight into a jug. The Ford engine uses an F-106N Fumoto Valve.
Search Results | Fumoto Engine Oil Drain Valves
Onan specifies changing the generator oil and filter annually, or every 150 hours. It should be done sooner in hot weather or dusty environments. It already has a drain valve similar to the Fumoto Valve installed at the factory. Onan recommends the first oil and filter change at 20 hours.