Once you get the battery stuff checked....
Some of your problems could be from you switch...but if the motor is running ok, then the switch is working ok. The cabling to the gauges from the switch could be an issue. The two things that loop from one gauge to the other is +12v and ground. If you have a volt/ohm meter, connect it to the two posts on the back of the voltmeter. Compare the two voltmeter readings. If they are both the same, the gauge is probably ok, if not, then you have a bad gauge. If both read the same but not around 12v, then problem is somewhere in the wiring.
If you do not have a volt/ohm meter, troubleshooting is going to be a guessing game.
Your fuel gauge problem could be a bad sender in the tank. Assuming all of the gauges are getting 12v, then I would look at the sender. Turn the key to on and momentarily short across the two terminals on the fuel sender. If the wiring, voltage and fuel gauge are ok, then the gauge will jump to the full mark. Just do it less than a second, so you do not hurt the gauge.
If your trim tabs are hydraulic, and one side works, the problem is probably just coincidental to the other starboard problems. If it is a Bennett system, check on their web site for a troubleshooting chart. It is very helpful. If it is confusing to you, you can call them and they will give you some basic ideas where to look for issues.
Mike