I've never seen a published weight limit on them, but 300 sounds about right. The only limit I've seen is for the fold down cabinet bunks @ 100 lbs.
Of course it's best to use judgement on full bunks- they are just made of wood, there's no steel reinforcement that I'm aware of. So if the trailer has any rot in the walls, the bunk won't hold up as much.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR