Well 14 and 28 is a good sign out of the alt... what do you see at the power panel test points on the left side when you open the dash power panel?) check with on and running... a bad high resistance connection resists current flow(like a kink in a garden hose) so with no current flow you should see the same voltage at the battery as is seen at the alt. That is why voltage is always tested under load... Properly designed a system should no more than 3% of voltage from no load to full load...
The fact that it is losing .3v from alt to batt indicates that what should be fully charged batteries, are pulling enough(minimal) current to the batteries that it can incur a voltage drop from the current restriction in the path(ground is part of the current path)...
You will see a .5 volt drop between batteries/alt and dash. The PPD has diodes that knock off .5v to whatever voltage it is being fed regardless of current flow thru the PPD...
These truck ar notorious for bad connections and grounds, like at the polarity protection device(PPD) between spare and air filter(lower spare to access) have you cleaned the 4 terminals on the PPD? Also clean the shunt terminals(below PPD) while you are in there. The shunt is in the cable between the battery ground and the ground point on the front of the starter motor(used to measure current to the standardized test equip connector under drivers dash)... These are out in the weather and are a regular issue...
The voltmeter simply samples the dash power, so if you have bad connections up to the cab, your volt meter is simply reading the decreased voltage in the cab due to the loads in the cab drawing current thru the bad connections... It's still in the green
All the grounds need overhauled starter to frame, alt to frame, pax frame to cab behind grill, cab to dash(under dash) dash to power panel ground test point(under power panel)... There are more associated with the lighting system in bumpers and rear drivers frame rail and crossmember(where pintle hitch is located)
The temp sensor is located on a T on the rear of the thermostat housing(above turbo), front right on the engine. you should be able to unplug it there and put in a jumper on the harness plug. lower and get back into the cab and turn on the ignition switch(only long enough to determine if the temp needle moves or not then turn it off). The jumper/low resistance should cause the meter to go to max temp. if it does, the gauge and wiring are probably ok and the sensor has failed. if the temp meter does not sweep to high, or even move, you have a wiring or gauge issue...
Good Luck!
