Fuel Sending Unit or Ground or some other electrical issue

[GeneP]

2000 Sonoma 4.3 4WD with 150K miles. Took a long 2,000 mile round trip last week. 1st note is about 1 month ago I finally had to replace the still original alternator, then everything was fine. on return trip this week I noticed what I thought was too good of gas mileage. 250 miles since last fillup and gauge still shows better than 3/4 tank. Before that all was well at 300 miles between fillups. Then someone pulled out infront of me while I was doing 75 MPH. Slammed on brakes to avoid collision, then floored it to pull around the idiot during my road rage event. The motor reved up to about 5K when I noticed the Battery Light came on. 1st thought I broke a belt but nothing over heated so kept going. At 70 MPH battery light stayed on but manually shift to 3rd and light goes out. Managed to keep going about 50 miles till I found an AutoZone and bought a new alternator. While in traffic in that town looking for a place to buy an alternator I noticed big time changes in the fuel gauge. At low RPM (Idle) it when to empty, rev the motor & it goes to 1/2 tank. Figured it was cause the battery was dead. Got the alternator, put it on in the AutoZone lot & again on my way, but the gas gauge is sill freaking out. Stays on full for 250 miles then freaks out intermittently until tank is empty. I'm thinking its odd that the new 1 month old alternator died at the same time as the fuel gauge. Maybe some ground somewhere???? I tried to get my arm up around the sending unit to look for a plug with a bad connection but arm is too fat to get up there. Any and all suggestions, recommendations would be greatly appreciated.

Gene