if I may .....

" checked ALL fuses which appeared good "
looks can be deceiving ...
everything under right
side cover as best you can
take'em off and clean'em ... one at a time ...all of it
wire brush and or emery cloth
under right side cover ... every wire , fuse , relay ...
and the battery cables + and - ... both ends ... oxidation and crud is the enemy

the cause of many disappointments , head ache and pains in the ass
little things first ... always

apply dielectric grease (is your friend) on each connection ... alternator wire ,,switches , light blubs ..all of it

$0.02 works for me

trailers are for boats ...