There is no BOS body in black mountain, the 3 locations where they are are Reppcon HQ, in the destroyed town in front of Nellis and east of the hidden valley in a large irradiated crater, the missing laser pistol is also east of hidden valley in an area called scorpion gultch at the bottom theres a boulder with a skeleton its on top of the boulder.
Thread belongs into the spoiler section but:
Yes there's a dead paladin in the area of Black Mountain.
Spoiler
The easiest way to find it is to step out of the BoS bunker in Hidden Valley and
head toward the fence by North/North-east where there's a opening and a path
climbing up. It will be infested by Bark scorpions and about half-way up,
you'll meet several Centaurs standing around irradiated craters and at
the bottom of one of such craters you'll find what you're looking for.
Cheers!