On this page of the guide to Mr Prepper, you will find information on how to get to Fort Observer.
Fort Observer is a mysterious location in the game Mr. Prepper. You'll only get there in the final stages of the game. Keep in mind that you can only visit this location once. In order to go there, you have to complete all the tasks for Bob, Jenny and Joe. These are completed by: getting sleeping pills from Jenny after meeting the insomniacs at Bob's, and investigating the signal for Joe in the desert.
When you meet these requirements, you will be able to refuel the car: you need 20 units of fuel, a lot of food for regeneration, hunger and energy. Do not take too much stuff with you: you will have to drop items other than food before entering the location. When you're there, however, you will have a short encounter with snakes and wolves, so it is worth to have some weapons.