2005 F-150 - Blown engine

2005 F150 5.4L 4WD. 46,000 miles. Truck had a rough vibration as I was coming to a stop one day. Later in the evening, started making a horrible noise, and sounded like a diesel 18 wheeler. Check engine light came on. Left truck at the nearest dealer. Mechanic told me there was very little oil in it, and engine was ruined. Said it needed to be rebuilt or have a new engine put in it.
My extended warranty (100k) will not cover it because I can not support all maintenance details of the truck, since I have done the majority of the oil changes myself.
Biggest question is, if the oil was that low, shouldn't the oil pressure gauge have registered a drop? Shouldn't I have had some sort of warning that it was low?