I assume you are talking about installing a 50 amp shore power receptical,
the receptical you need to install on the outside of the bus is a 14-50R, it looks like a 240v receptical but it is not it only has 120v running through it, so dont let it get wired wrong or it will fry everything you plug into it.
From that outside receptical to the breaker panel you probably want 6ga wire,
From the breaker panel to your wall recepticals and lights 12g is best for up to a 20 amp receptical,
14g wire is ok if it is a 15amp receptical or less.
my bus thread, https://www.skoolie.net/forums/showthread.php?t=8860&highlight=bubb