Fussing with the VesselView warnings has taken a bit of time (see my "Mercruiser 6.2 ECT" thread). As such I have not made much progress on the other fix-its and getting things dialed in on this boat.
I've joined a FaceBook group for the Cruisers 328/338 and that has helped with some ideas. Here is a quick update on some of the things . . .
Trim Sender (starboard and port)
I've purchased a new trim sender, but just connected it inside the engine bay, not mounted to the gimbal ring, etc. Of course it does not 'work' (show the true position of the outdrive), but it keeps the VesselView happy. I've gone for many a season with little or no functioning trim gauge(s).
My plan for the trim senders, since they are $150/each and may last only a few years in the water, is to make/connect a mechanical cable system to the trim cylinders and have that connected to a lever arm inside the engine bay that 'turns' the trim sender as if it were mounted on the gimbal ring. I figure that if I can keep the sender dry by not having it in the water, it may last a long time.
I did something similar on my previous boat, but those senders were a simple resistance device. These senders (3 wires) are more complicated and I have not been able to figure out what is inside by probing the leads. It seems like a combination of resistors and diodes, but who knows?
Restroom (head) Lighting
As for the restroom . . . which does not have a natural source of light, I have a PIR switch and an extra LED light to add to the mix. So, folks using the rest room will not have to worry about finding the light switch once they close the door and are in total darkness. The is plenty of wiring to tap into, so it should be a few hour job installing everything.