Generally, gauges that peg full have bad wiring at the gauge or a wiring issue somewhere in the circuit between the gauge and sender. Of the three gauges you mention as pegging full with the key ON, two of the three require a sender which is a variable resistor, the position of which feeds a variable voltage to the gauge which translates that voltage into a fuel level or trim level. The battery gauge (a voltmeter) does not need a sender and merely measures electrical system voltage. With the engine off but key ON, that gauge should read static battery voltage which is 12.6 volts give or take a couple of "tenths" depending on the state of charge. With the engine running at slightly above idle, it should read 13.5 - 14.5 volts (again give or take a bit). Those gauges typically have about 18 volts on the high end of the range and if the needle actually pegs at 18 volts with the key on (engine running or not), something is really weird because you cannot get 18 volts from a 12 volt system with the engine OFF/key ON. I'm guessing, but I think you have a ground problem involving those three gauges. I would start by disconnecting either the +12 volt or ground wire on the voltmeter as a starting point. See what happens. Mark those wires so you can put them back on correctly. Leave those disconnected and then disconnect the wire on the "S" terminal on the trim or fuel gauge. Again see what happens. Again -- mark the wires. With those three gauges disconnected, re-connect the voltmeter. Turn on the key but do not start the engine. Does the gauge now read correctly (12.6 volts)? If yes, that gauge is not the problem. Re-connect either the fuel or trim gauge. Does the voltmeter peg again or does it remain correct? If correct, does the gauge you just connected work correctly. If not, disconnect it again and re-connect the last gauge. Does it work correctly. If so you have identified the gauge and circuit with the problem (it was the middle/second one). If the first two gauges you tested work correctly and the last one failed then that circuit is the issue. Unless you provide more detail there isn't much we can do so you need to look carefully at the wiring both at the gauges and at the senders (fuel and trim) which are in the obvious locations at the back of the boat.
