what is wrong when the voltage battery gauge in dash fluxuates when u drive or idle then when u turn the truck off instant no power left with consideration that it only needs to be on a minute for this to happen and doesnt change even for 3hrs like that it has full power till turned off i got a 94 ford f350 xlt 460 7.5l v8

The battery terminals are subject to corrosion. You can't even see it because it's on a molecular level, but what is happening here is the classic "brownout" where varying amounts of power can get thru. Probably is a CLUSTER of connections at the positive terminal. Brass wire brush and use some electrical cleaner spray. Do not forget the ground lug attachment. Clean up, and all will be well with the world.