I agree the problem is low or no 12 volts.. How it works
120 volts from house/garage comes into the RV.. NOTE: some RV's will trip GFCI by code all outdoor outlets are GFCI or GFCI protected so make sure there is power at the outlet (plug in lamp)
now it comes into the RV. if it's a Trailer it USUALLY goes straight to the breaker box but in a motor home there is a transfer switch (or a trailer with optional factory generator).
IF the CONV breaker or main is tripped.. No joy..
Using that lamp. Find an outlet that is NOT inverter powered (If you do not have an inverter that is every outlet in the house) and plug in NO light = No power.
Check GFCI. there is likely one in the bathroom. IF there is do a test and reset. it may refuse to reset if no power.. Plug in lamp there.
Now from the CONV breaker power goes to a box called a CONVERTER (CONVerter) this may be right behind the breaker box or it may be... elsewhere.
This box converts (Converter) 120vac to 12vdc (Battery) it has 1,2 or 3 30 amp fuses which may be blown. (if more than one they will be side by side).
IF you need to replace them.. UNPLUG replace all of them Plug back in.
From there it goes to the house system. then to the battery disconnect. and via a fuse or breaker to the battery when plugged in.
WHen unplugged reverse last paragraph as far as "house system" and stop.
IF you have an inverter/charger.. Small ones same as above
Big ones are often connected direct to the battery via a fuse or breaker BEFORE the switch.
Now on my RV (Which is of course the one I know best) The CONVerter is not behind the fuse block and is a PLUG IN device in an outdoor compartment. Sometimes it becomes unplugged (Very easy fix).
After 12.5 years it died (Also a very easy fix but 238 dollars).
And as I said some folks have blown fuses. Mine did not.