Everything on 12 volts is dead. No lights, no pump, no fans, no control panel. The shore power breaker has not tripped and the 120 volt outlets still work.
That combination is telling you something specific: the coach's DC side has lost its supply somewhere between the battery and the distribution panel. It is almost always one of six things, and you can work through them with a multimeter and twenty minutes.
Work in this order. Each step rules out a section of the system.
1. Is the battery disconnect switch on
Obvious, and it catches people constantly, because there is often more than one.
Many rigs have a battery disconnect near the battery, another inside near the entry door, and some have a separate one for the chassis battery. A switch that has been knocked, or left off after storage, kills everything downstream and looks exactly like a failed component.
Check every disconnect you have before touching anything else.
2. Is the battery actually charged
Put a meter across the battery terminals.
- 12.6 volts or above: the battery is fine. Move on.
- Around 12.0 volts: deeply discharged. It may still run some things, and low voltage causes odd partial failures rather than a clean dead system.
- Under 11 volts, or near zero: either the battery is flat, or its internal protection has disconnected, which is common on lithium after a deep discharge or a low-temperature event.
Measure at the battery posts, not at the cable lugs. A corroded or loose lug will read correct voltage at the post and deliver nothing to the coach.
3. The main fuse between battery and coach
There is a large fuse or breaker between the battery positive and the rest of the system. On many rigs it sits within a foot of the battery, sometimes inside a black plastic housing that does not look like a fuse holder at all.
If this has blown, nothing downstream works and everything else tests fine. That is exactly the symptom you have.
Test it with the meter set to continuity, or measure voltage on both sides. Voltage on one side only means the fuse is open.
Never replace it with a higher rating to stop it blowing again. That fuse protects the main cable, and the cable is what burns if the fuse is too large. If it blew, something caused it, and the cause is usually a short somewhere downstream.
4. The reverse polarity fuse
This is the one most owners have never heard of, and it is a very common cause of a totally dead coach after someone has worked on the battery.
Many converters and distribution panels include a reverse polarity fuse, sometimes a pair of them, sitting behind or beside the main fuse block. Their job is to sacrifice themselves if the battery is ever connected backwards, protecting the converter and the coach wiring.
Connect a battery backwards for even a second and these blow. So do they, sometimes, from a jump start done in a hurry or a charger clipped on the wrong way round.
Where to look: open the distribution panel and find the converter section. The reverse polarity fuses are usually large blade fuses mounted separately from the neat row of branch-circuit fuses, often on the converter's own board. Your panel's label or manual will name them.
They look intact far more often than they are. Test them rather than eyeballing them, because a blade fuse can fail with no visible damage to the element.
5. Grounds
A DC circuit needs a return path, and the coach ground is easy to overlook because nothing about it looks like a failure.
Check the battery negative cable where it lands on the chassis. Look for corrosion, a loose bolt, or a ring terminal that has been painted over. Clean to bright metal, remake the connection, and protect it.
A bad ground produces stranger symptoms than a bad positive. Things work weakly, or work only when something else is switched on, or the whole system behaves differently in the wet. If your fault is intermittent rather than absolute, look here early.
6. The converter, if you have no battery
If the coach only fails on shore power with no battery connected, and works fine from the battery, the converter is the suspect rather than the distribution.
A converter that has failed will not power the coach on shore power and will not charge the battery, so the battery slowly flattens and you eventually get the dead system you are troubleshooting.
The fast version
- Every disconnect switch on?
- Battery reads above 12.4 volts at the posts?
- Main fuse or breaker passing?
- Reverse polarity fuse passing?
- Ground clean and tight?
- Voltage present at the distribution panel input?
If you have voltage at the panel input and still nothing works, the fault is in the panel itself rather than upstream of it.
Making it easier next time
Most of the difficulty here is access. Fuses hidden behind panels, connections buried under furniture, and no way to isolate a section without disconnecting the battery.
Three things worth fitting while you have the panel open:
A resettable main breaker instead of a fuse. A 50 A, 200 A or 300 A manual-reset DC breaker gives you a switch you can operate and reset by hand instead of a fuse you have to carry spares for. They are waterproof, which matters in a compartment that sees road spray.
A proper distribution point. A busbar or a pair of terminal studs replaces a stack of ring terminals on the battery post with something you can actually inspect and test. Stacked rings on a battery post are a recurring source of exactly this fault.
Fuses you can get at. A MIDI fuse in a proper holder mounted where you can reach it beats a fuse zip-tied behind a wall.
Size any of these against the cable they protect rather than against the load. Our DC cable and fuse sizing guide covers the method, including the interrupt-rating rule that matters if you have moved to lithium.
If the fuse blows again
Replacing a main or reverse polarity fuse that immediately blows again means there is a genuine short. Do not keep feeding it fuses. Our guide to a 12V fuse that keeps blowing covers how to find it.
If you have worked through the list and still have no power, send us what each step measured and we will help narrow it down. Everything ships from Upland, California.