the 'black' thing is a power plug, connecting something in the RV
the fuse holders at the bottom of the picture, are a 'set' of parallel fuses
probably going from the charge converter to the battery
unplug, unscrew and clean all those connections, clean the wire terminals, get new screws or clean those, rust/corrosion causes resistance, resistance where a battery is charging, creates heat,
check your battery, take it to an auto parts store and have it tested
a shorted cell, will create high current and lots of heat