Will not start on incline

Just to be sure, this vehicle will start if it is on level ground but will not only if it is only an incline? Does it matter if it is nose up or down?

Here is a guide that will help with this issue but I suspect the issue is the fuel as you are suspecting:

https://youtu.be/dCjmRL3p4Cs

After you added the 5 gallons did you move it to flat ground and it still not start?

If it is still on the incline then let's try and use starter fluid in the intake and see if it fires for a second or we can check the fuel pressure using this guide.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Let me know about this info and we can go from there. Thanks