My battery voltage is displaying the wrong value. It's always hovering around 11.5 to 12.5v when the actual voltage is 13.5 with the engine running.
Also the transmission temp is reading very high.
The difference between any two points in the coach will be measurable. Very good question: Why is _this_ so different and how do I fix it? A little background, then the solution:
The voltage seen on your VMSpc display is one being reported by the ECU. Its point of reference is its own very supply of power, as delivered by the Ignition Solenoid. If the solenoid is delivering 13.8 but after traveling criss-cross-applesauce through the coach wiring drops down to 11.5v then there is a real problem alrighty. The ECU will call foul, 11.5v is a problem. Easy enough to understand yet tedious as heck to trace.
However, if the voltage delivered to the ECU _is_ 13.5Vdc but its reporting 11.5 then the solution is to zero in on the ECU; it's misreporting the voltage internally. Not so tedious at all, just danged expensive.
Yet of these two scenarios, one ...or both...is the problem. So, to take things from theory to your fingers; I'd pick the solution with the highest probability: bad wire from the solenoid. I'd grab an old shirt, voltmeter and a bottle of NoAlox, nitrile gloves, scraper and some wrenches (3/8"~3/4")...flashlight... and coffee. Gather up all that and start at the output terminal of the solenoid. Be prepared to break, clean and coat the connections along the way. This goes for the grounds, too. Look for voltage drops.
Odds are that its a bit of scale or oxidation and easily enough fixed but for the crawling and stuffing yourself into some really icky and cramped spaces.
Yet that...is the solution on this Fall Thursday.