IIRC it's about six or so quarts; if you get two five-quart jugs, you'll have plenty and some left for the next oil change. There should be a Ford owner's manual for the chassis with the motorhome that has details such as that.
Just confirm which chassis you have. Most Ford c-class MHs were built on the E-450 cutaway chassis with the 6.8L V10. (not the F450). Here is some info for the 6.8 V10.
CAPACITIES AND SPECIFICATIONS - 6.2L Capacities WARNING: The air conditioning refrigerant system contains refrigerant under high pressure. Only qualified personnel should service the air conditioning refrigerant system. Opening the air conditioning refrigerant system can cause personal injury. Item Capacity Engine oil (with oil filter). 7.0 qt (6.6 L) Engine coolant. 21.3 qt (20.2 L) Brake fluid. Between MIN and MAX on brake fluid reservoir Front axle. 2.7 qt (2.6 L) Rear axle-10.5 inch with Electronic Locking 3.3–3.5 qt (3.1–3.3 L) Differential. Rear axle-10.5 inch without Electronic 3.5–3.6 qt (3.3–3.4 L) Locking Differential. Rear axle-Dana M300. 3.78 qt (3.58 L) 1 Automatic transmission fluid (6R100). 13.9 qt (13.2 L) 2 Automatic transmission fluid (6R140). 17.4 qt (16.5 L) 2 Transfer case fluid. 1.9 qt (1.8 L) Windshield washer fluid. Fill as required Fuel tank (incomplete vehicle, middle 26.5 gal (100.3 L) location). Fuel tank (All except 176 inch wheelbase 34.0 gal (128.7 L) and incomplete vehicles). Fuel tank (176 inch wheelbase). 48.0 gal (181.7 L) Fuel tank (incomplete vehicle, aft axle 40 gal (151.4 L) location).