So I've completed the Lonsome Road, as well as the other DLC's and am at level 43. Now, to get to 50 I need a large amount of experience, somewhere around a total of 500k for level 50? I currently have 142k exp. Now I still need to do some exploring within the Divide, and a little bit through OWB, but I doubt that's going to get me over 350k exp. So, where should I go to farm enemies/level up to 50?

A few areas with respawning enemies off the top of my head -

The ruins southwest of the Strip - Fiends
South Vegas Ruins (where you enter Vault 3) - yet more Fiends
Gulch north of Goodsprings, desert south of Nipton Road - Radscorpions
Bonnie Springs, Tribal Village, Callville Bay - Cazadors
Plains west of I-15 and south of Primm - Ghouls
Outside of Vault 34 and Gypsum Trainyard area - Golden Geckos and Deathclaws
El Dorado Dry Lake - Fire Ants
Powder Ganger Camps and outside of Jean Sky Diving - Powder Gangers

There are plenty more.

doing the quest up to the point right before you enter the casino to confront bennie should give you the rest to hit 50.......