Your payload rating is based on your truck’s trim and options unless you’ve added aftermarket stuff.
Mountains don’t matter if you remember physics.
If you ever get worried, just slow down a bit or lower the heat a couple of degrees. You’ll be fine.
Elevation isn’t the issue—it’s about energy use.
Climbing kills efficiency, but regen gets a lot of it back. You’re doing pickup stuff in a pickup.
80 miles round trip? Charge up if it makes you feel better, but you’ll look back and laugh at how easy it was.
1400 feet is a mountain now? 
The energy needed to lift a weight X up Y feet is well-documented physics.
I ran the numbers:
You’ve got 127 kWh in your battery pack. Using just 1 kWh will lift 1200 lbs up 2215 feet.
So yeah, you’re fine.
Just don’t go downhill with a full battery, or you’ll lose regen braking. You don’t want to cook your friction brakes on a long descent.
Is the compost on the mountain? If so, you’ll get free energy on the way down. 
I do 75 miles towing a tritoon deck boat (basically a parachute for range). 1200 lbs in the bed is nothing.
1200 lbs in the bed? Barely makes a difference unless you’re trying to go 85 mph the whole way.
Aerodynamics matter way more.