To check charging voltage from the truck, with the + lead of the meter, touch the positive side of the battery ( it can also be disconnected, but always disconnect the negative side first) then touch the negative lead to the ground wire. As mentioned above, you might have a loose or corroded ground. Use the ohm meter function to check that. If you’re uncertain how to go about it, I’m pretty certain YouTube will have tutorials on troubleshooting, and use of a multimeter.
edit: your fuse may be masquerading as a thermal circuit breaker that has tripped. Where ever it is, it should be, but may not be, close to the truck battery.
Another thought that may take us down the rabbit hole. Can you tell where the charging wire goes in the camper? Is it straight to the battery, or do both connect to a positive bus, where other the devices get their power. Any fuses or circuit breakers around the bus. Lastly, is your camper battery switch on and working.