It seems to me that either the fuse is mislabeled or that the courtesy lights and cruise control are on the same circuit. The comment that the battery would run down if the courtesy light fuse is left in sounds like hogwash, unless there is something else on that circuit that is always on or you turn on something that is part of the chassis electrical system.
I don't own a motor home but, to the best of my understanding, there should be a battery isolator in a motor home that disconnects the engine battery from any load imposed by by the coach when the vehicle is not operating. All loads from the coach are then carried only by the coach batteries. Once the engine battery is isolated from the coach circuits there should be no active load on the engine battery unless you turn something on that is wired into the chassis circuits. Examples might include the driver's compartment courtesy lights, the dash radio, headlights, running lights, etc - same as on a regular 450 van. I have no idea if the isolator on your coach operates automatically or not when you turn off the ignition or, for that matter, if they were even standard equipment on an '03.
Happy Camping! ///// Richard D.
2006 4x4 Ford 250 SD / 2007 Flagstaff 827 FLS
One very patient wife and one furry child who travels with us. Forty-two years of trailering and camping, and I still have a blast.