The voltage output on my truck is 18 volts. I had the alternator checked and replaced the regulator. Still 18 volts. Any ideas

Dash cluster is out a waack

Look for a corroded bulkhead connector on the alternator circuit. Nasty one to find.

Just a question?

Is this 18V from a dash meter or from a multimeter at the battery?

A quick check also is run a wire from your alt housing mounting bolt to your battery -----GROUND ----clamp. Make sure wire is not routed near any rotating or hot parts. Check at battery to see if it settles down.

Like KISS said bulkhead and also check your ground strap to the frame!

Make sure your voltage regulator makes good ground with the firewall. Remove regulator from firewall and use sandpaper to clean back of regulator and firewall. Screw back in place. Check cab is grounded. Ground straps corrode after many years. Also check if getting voltage to and from regulator. I had a break in the copper inside a wire to the regulator, wire looked good on the outside. Regulator wasn't getting a signal to stop charging. Old Dodges have reliable charging systems. I still drive them. Carry a spare in the glove box along with ballast resistor and ignition module. Can even install spares under the hood and switch wires when you have to.
Keep on trucking.