Trim and tach gauge not reading properly?

[Mongoose4thdan]

About 2 months ago my trim gauge stopped reading properly(it would not read all the way down, it would only go about 3 bars from the bottom,but the physical level of the actual trim was working fine. On my last trip out, the Rpms started reading funny. When I would engage the motor and trim down at the same time , the rpm will read zero. If I just engage the motor or even when I trim up, the Rpms read properly. Any suggestions.

[boats.net]

Check all wiring between key switch and gauges and between gauges and engine, check all ground connections on engine and gauges. Check battery cables and connections.