It really sounds like a power cable issue.
Measure the Battery Voltage with a voltmeter. If you don't have one, beg, borrow, or steal one.
If the battery's dead, take it in and have it charged/tested.
If the battery's good, you're going to have to do some diagnostics, following voltage and ground up to the ignition switch, and back to the engine.
Because the starter turns, you have 12V going at least as far as the ignition switch, and back to the starter solenoid. That's good.
When the gauges go dead, check an unswitched power accessory like the blower, bilge, nav lights, courtesy lights - do they work? If so, you have power getting to the dash, and likely have an issue with the switched power coming from the ignition switch.
There's a (IIRC) 10-pin connector on the front of the engine that goes to the cockpit. This connector carries all the voltage for all the dash and the ignition switch; you should notice a very big (8 or 10 ga) black wire and red wire coming from the connector. Wiggle it, and make sure its well connected.
The wires from this connector are connected to another 10-pin connector under the dash; on mine, the connector was on the backside of the footboard in front of the driver; you can pull it through the hole in the footboard (it's the biggest clump of wires). Wiggle it, and make sure it's well connected.
After this, things get complicated. Wires go everywhere. In general, 12V goes to the ignition switch. When in "Run", the purple wire should get 12V, providing power to the gauges and goes back to the engine to enable the engine computer. This wire goes everywhere, and run everything. It goes through the kill switch; your symptoms exactly match a bad kill switch. Find the connector under the dash with two purple wires, and jumper the wires together. See if things get better.
Good luck; let us know what you find.
1998 Maristar 200VRS