You must first complete the Fort Mercer mission and then the bridge to Mexico opens and you can then go there.
Because undead nightmare is a seperate scenario from the ordinary red Dead Redemption and they are both single player.
you cannot have posse's in single player mode, only multiplayer
In multiplayer yes, single player no
you do not level up on single-player free roam on Red Dead Redemption.
You Don't
you can only get golden guns in multiplayer mode
yes it should be on when you press single player
Red Dead Redemption is set in Mexico and America. Camels live in Africa, India and parts of Asia therefore there is no camels in Red Dead Redemption.
do your missions
eastern part of mexico
in mexico
Yes but you have to kill that fat cowboy dude outside the salon!