That's a big, heavy, rolling brick of a vehicle. You're going to want something much beefier than a 5w30. That being said, a Mod motor will be happy running on basically anything you decide, from dino 5w30 to synthetic 10w40.
What kind of conditions will the RV see while in use? Hills, ambient temps, loads (water, waste, passengers etc).
Might I suggest this: Valvoline Premium Blue Extreme 5w40
Premium blue gets big thumbs up around this forum, and the extra protection of running a xw40 won't hurt anything. I would use a Fram Ultra filter, it's one of the highest rated filters on this board, as well as affordable.
Does the rig have an oil cooler? I believe on the mod's it's just a sandwich cooler between the filter and block. It's more of a warmer than a cooler, but if you aren't pushing 70 through the Nevada desert in August you should be fine.
There should be a 4r100 trans backing up that motor. Solid unit, definitely install a pan with a drain plug (if not equipped, many commercial chassis were from the factory) to make drain and fill's easy. Have the spark plugs been done? I believe '99 is still within the years that had too short of spark plug threads and tend to loosen up. I would recommend having all 10 done right now if they haven't been, and make absolutely sure they're torqued to the UPDATED spec from Ford.
Let's see some pics, BTW!