Ok, there might be an alternator issue in the truck that only really shows up when doing this, but it does hold the truck voltage up ok around 14v (14.5 or so when first starting) Truck voltage holds up ok with driving all day on 7-pin, but that is at low amps of course.
My dash battery lamp that is bright but then goes out when you start the truck never goes completely out. It stays dim red all the time instead of going right out. Been like that since the truck was new in late 2002.
So the thing to try is put the jumper cable pos clamp on the alternator pos lug and the jumper cable neg clamp on the truck frame where the alternator bracket is instead of on the battery posts?