According to my key list that Airstream provided in their web site data, the 1977 keys are these:
Main door: This is a KT lock, and if original, will have to have a new cylinder or if the cylinder is OK, a key made by a locksmith. This is the only lock with a trailer specific key in 1977.
Trunk door and access doors: 1101X key. All doors, all 77 models. Available at your dealer, or Out Of Doors Mart or probably Inland RV. Also if you can find anyone with a '77 trailer, the key will be the same, and you can have one made as they are all identical.
Battery door: LL432 Same deal as above, all the same for this year.
Water door: CH751 Same deal as above, all the same for this year.