Hi bobkatbry, welcome to the vintage Airstream community. I have a 66 Trade Wind and it is a bunch different than your 76. Back in 66 the factory installed a drain valve and line along the water tank plumbing to the 12v
water pump. My tank is above the floor under the front gaucho.
But I have used the water pump to drain (most of the water) out of my fresh water tank for years with no problems. Your trailer may have a "low point" drain valve in the plumbing system. Look for a hose exiting the bottom of the trailer somewhere. (You may have an air conditioner condensate drain hose under there somewhere too.)
You mentioned you don't have a key for the fresh water inlet door. Neither do I on my 86 Limited. But this "inexpensive" lock is easily picked. I use a very small screwdriver through the keyhole and feel the tumblers. I lift them, and then rotate the lock. I've been doing this for 10 years as I never felt the need to replace the lock. This might work for you.
I always "bleach" the fresh water system in the spring and / or before a trip. I add a half cup of bleach to the fresh water tank, pump it to each faucet (kitchen and bath) and then let it sit overnight. This will kill any unwanted bacteria. Then I drain, fill, flush, and repeat until I don't have a chlorine taste in the water. I have read others do this too.