I have a 14 prostar with the N13 engine. Yesterday I stopped and ate and was out of the truck for about 8hrs. When I came back to the truck it wouldn’t start and acted like dead batteries. I had a mechanic jump start it with no issues. I went in to take a leak came back out and it was #### off so I tried restarting it and it acted dead batteries again so I called the mechanic and had him jump me again. I got it running and rolled out. I made it 10miles down the interstate and the volts started dropping and the truck died. So I called the mechanic back and we figured it had to be the alternator. He didn’t have the alternator but he had a 4 bolt for a freightliner so we installed it. We jumped it and it started up and ran great with the cables connected but again we disconnected them and it died. We couldn’t figure out what is wrong so we started checking wires and grounds. Cleaning contacts ect. We started it up and ran great so to get it off the interstate we put a battery he had in the floor inside the truck and ran jumper cables to the batteries so we could keep enough power to get to the truck stop and it worked. When I parked it and disconnect the extra gel battery we used it stayed running and charged at 14 volts so we just figured whatever wires we messed with must have fixed the problem. It idled all night with no problem and holds 14 volts. This morning I tried to pull out and it’s in derate mode and won’t go over 5mph so I hopped and jumped all the way back to the parking spot and it’s here idling just fine but won’t drive over 5mph. Any idea what the problem is or what to do or check before I leave it at the truck stop and tell the bank to come get it?