Your 10 dollar digital volt meter from Harbor Freight is your friend, (Or it's equal, or superior) in fact a test lamp will do if you must (I rather like test lamps).
Measure the battery voltage AT THE BATTERY, 12.6 resting, 13.6 if plugged in, GOOD.
Now move the black lead (NEGATIVE) to an exposed part of the RV frame, same reading Great. Less, bad ground connection. either end of cable.
Now let's move out along the wire, this is where the test lamp really shines,,, or not (If it shines good, if not you found the problem).
With reference to any handy frame point, check for power.
If you come to a small metal cube like thing with two bolts, nuts and lock washers, test both, odds are only one has power, that's a self resetting thermal circuit breaker that has failed, get a new one.
If you come to something that looks a bit like a Ford Starter solenoid, that is a disconnect solenoid and they have two failure modes.. one is bad solenoid, the other is SUE, that's Stupid User Error (Forgot to turn it on) Though in fairness when mine failed *I* turned it on (or tried to) Switch was bad (Fixed it next day).
And so on,, But that is how you do it.
Of course if the batery itself shows no voltage,, Problem located.
Home is where I park it!