I also doubt that the fuel lines are old enough to have cracked, Dave. That's not to say a fitting may not be loose. But there should be no fitting leak at 3/4 tank, regardless.
I was finding some leakage on our pad nearly under the tank a few years ago. Forgive my dimming memory, but I recall there is a vent tube, apparently from the fuel tank, that winds forward over some framing, and I thought it was from that, since everything around was oily. I try to always park the coach at home with a full tank, and figured it was fuel expansion in warm weather out a vent tube.
But I eventually determined it was probably from when I'd recently had the generator serviced; the tech hadn't properly tightened the filter. Driving down the road caused a very slight leakage to "spray" over time back over the fuel tank area and adjacent parts, then when I parked, it gathered and dripped to the ground. What appeared to come from the tube was in actuality coming from the generator immediately in front of the tube end. As I recall, there was a fitting or housing under the generator filter acting as a catch basin of sorts, so the puddle wasn't directly under the filter.
I'm not sure you'd get anywhere near a gallon, or even a cup, out of such a scenario, but just a thought for you to check.
Joel