The trim sensor is on the right side of the outdrive and plugs into the pivot bolt for trimming the drive up or down. The sensor is a rheostat, and is prone to issues. It is $130 or so on eBay. It can be a challenge to change, especially where the wires go through the transom. This should be a url address for a service manual for the outdrive.
http://boatinfo.no/lib/volvo/manuals/sx_dp-s_drive.html The trim system uses a power steering fluid. The trim system and the sensor are separate systems.
The Four Winns boats, and some other brands, have been accused of using lighter wiring than needed. Voltage drop is a common complaint. You could try a temporary ground wire to see if that helps. My 268 volt meter readslow also. The instrument panel is held in place by 6 screws and can be removed while sitting in the captains seat. The tachometer may eventually start to read high and be erratic. A tap on the gage may bring it back in line, but moving the cylinder selector switch on the back of the tach back and forth usually cleans the contacts and makes it work right.
If you do an inverter, I recommend a 2000 watt True Sine Wave unit. Keep in mind that a 700 watt microwave will draw almost 7 amps on 110 volt, but that means it will draw 80 amps or so from your 12 volt battery bank. This works for a few minutes but draws a lot from the battery. We don't bother to use the inverter for the cooktop or the water heater. We do use a Honda 2000 with good results. The built in CO detector system may not be operating correctly after several years. New replacement units can be obtained for $85 each or more. Regular household CO detectors may be a more attractive option at approx $30 each.
I'm not sure where to get the light. I would recommend your local Four Winns dealer.
Enjoy the boat and welcome to the forum.