I don't have numbers for the overhead bed, but the ladder has a weight limit label, and if I recall, ours is 225 lbs. The overhead bed is designed for two adults; I would suspect it'll handle at a minimum 2x the ladder load + mattresses & other items. I wouldn't be hesitant to put 500lbs+ up there.
Regarding the propane tank - Look on the tank itself - there's a metal label welded onto the tank. This will tell you the gallons.
Formerly: 2000 Four Winds 26Q/Ford, '96 Kit Sportsmaster 22' 5th wheel, '91 Sunlite truck camper, Wildernest flip-top canopy