Doc is not thinking fourth-dimensionally!
There are SO many options to their 'gas problem', it boggles my mind that someone supposedly intelligent can't figure it out.
Many people have already talked about the cave DeLorean and it having had gas and Doc having stored it somewhere - there's NO way this wouldn't have happened, from what we know about Doc's meticulous character.
Any other sequence of events wouldn't make any sense.
However, let's put all that aside for now, and think about another solution and possibility.
Time travel is always a bit convoluted, because everything influences everything, and thus, even the most intelligent writers in hollyfjord just can't think of all the possibilities.
Now, we know Doc 'wrote' (NOT in a past tense for time travelers!) a letter to Marty.
Everyone's head should be louder than a church bell tower on a sunday morning right now.
Doc... sent.. a letter to Marty in the future - his current past.
Why is this, among other things, considered as 'having happened', just because it was already shown to us in the movie? Chronologically or fourth-dimensionally thinking, the letter is still on Doc's table, or Doc can revise it, ask it back, you name it. The letter is in their vicinity, they have access to the very letter, so they can easily MODIFY it.
What if Doc and Marty together modify the letter to include things like: "Please bring gas with you", "do not use the movie theater parking lot, because there WILL be indians that will destroy the car's mobility, or at least choose night-time for arrival time. There's also a bear in the area, probably in a cave, so make sure to avoid such areas" and remove the 'don't come to get me' part?
They could write a full manual on how to avoid Biff altogether, avoid ending up with any bad situation and so on. But nope, because it's more intelligent to go with the hare-brained train-plan.
This movie's makers certainly weren't thinking fourth-dimensionally, or they would have REALIZED that 'always in motion is the future', things are not written in stone just because they have already happened in the movie - they could still be changed to be completely different for the characters.
Don't believe me? This movie and the previous movie couldn't EXIST if what I said wasn't true. If a matchbox can change from Biff's Pleasure Paradise to some Auto Dealership or whatever it was (so many problems with this, too, because how would Marty get his hands on a matchbox from an auto dealership or whatever car wash it was? HE WAS NOT THERE! Also, the same font for two completely different establishments, one of which was founded by 'Rich Biff'? Also, the same exact matchbox design and everything and and and..)..
..surely a letter from Doc to Marty can ALSO change.
RIGHT?
If not, why not?
Just to re-mention it, as one does, why isn't it enough for young Doc to simply _KNOW_ all this? Nothing changes future more effectively than knowledge!
Also, would Doc _NOW_ really tape the letter Marty wrote to him about being killed back together? Now that he has been revisited by Marty and helped him time travel into the past to try to save his future self that doesn't HAVE to make that choice that makes him end up in the past..?
I mean, wouldn't that seem a bit redundant now? Wouldn't his attention be diverted? Besides, after KNOWING he will be shot in the past and causes all this time travel mess and such, wouldn't he just at this point simply make the choice of NOT inventing any time machine, and thus preventhing the whole mess from happening?
(I know it would lead into a paradox again, but that's time travel combined with bad, thoughtless writing for ya)
To add a bit of a sidenote; why is Doc hell-bent on fixing Marty's future, when it comes to the whole 2015 family disaster, but also do not want to even tell Marty about his future when it comes to the car accident? Why the inconsistency? Why not simply fix every problem in Marty's future that would destroy his life/family/career/etc.? Why ONLY focus on this one problem? It doesn't save the future.
Also, KNOWLEDGE AFFECTS FUTURE.. so all Marty would have to do.. and also, why doesn't future Marty KNOW to expect Doc after he made Doc promise to visit him in the future? I mean, Doc wouldn't have to avoid future-Marty at all, he could just say 'well, here I am, as promised', and they could have a cup of coffee.
To add a tiny point, how does that Tombstone photo make any sense? So, someone took a photo of an empty tombstone? Why would they PUT an empty tombstone in a graveyard? That someone also took a photo of an empty patch in a graveyard? Wouldn't there still BE a tombstone there anyway, because people DO DIE CONSTANTLY, and why would they just leave one graveyard spot empty? It would just be replaced by someone else, right?
Who was that mysterious photographer? Don't tell me it was the Doc, because what motivation would he have to DO that? Film was expensive and photographing very cumbersome in the fifties.