As Paul notes, the fuel pickups for the genset and any hydronic heat system will be placed well above the engine fuel pickup to make sure you can't strand the coach somewhere. If it's been parked/stored, are you sure there isn't a static tank leak? The hydronics and generator use roughly 1/2 gallon per hour, so you'd have to exercise them a lot to burn up 84 gallons (75% of 112). I exercise ours every few months, and some contend that the hydronic exercise isn't necessary.
Generally, it's advised not to run the engine at all if parked for long stretches unless you can get it up to full temperature. And that doesn't work the braking systems or other highway components. I've had some input to the contrary: run the engine occasionally to ward off corrosion. Opposite suggestions from two highly respected BAC sources.
Joel