What engine ya got in yer rig ? Yer symptoms sound like a gasket and/or head problem.

If you don't have the optional clock, or power windows, there won't be a fuse or relay, but the wires are in the harness.

I'll take a look at the schematic and see if I can give ya any ideas on the gauges... I think someone recently asked this, but I don't remember for sure, or what the answer might have been...

Does the OIL PRESSURE gauge appear to be working properly ?

I don't see anything common to both gauges other than the regulator circuit on the gauge assembly, and that also powers the OIL PRESSURE gauge, so if that's working, then the regulator is probably OK. I think you may have two seperate problems with the guages, either wiring, or the temp sensor is bad, or the fuel gauge sending unit may be bad.

Cylinder leak down test is probably best for diagnosing head gasket failure. Compression check will give some clues.