I've got the Fluviacarte guide for La Seine Aval (Le Havre to Paris), by all accounts it's the "bible" for this route but I keep hearing the info on facilities is inaccurate.
From what I've gleaned from hours of net searching is that fuel is reasonably available quayside in the non tidal section (Paris-Rouen) PK0 to PK 242. PK, or "Point Kilometre" is the distance in km from the Paris lock near Notre Dame. The open sea is at PK355, Rouen is at PK242.
So fuel appears to be available at PK81 Marina Port St. Louis, PK121 Port L'llon and PK 241 Rouen. Also in Paris itself, 3 or 4 clicks upstream of the lock, near to the Paris Arsenal marina, the main place to stay in Paris.
Downstream of Rouen there appears to be no fuel until Le Havre which is about 90nm, this would seem to be the longest leg without fuel, all needs confirmation though.
I've just spotted a throwaway comment in my guide about locks being closed at Easter but can't find anymore on this so who knows
I can see this is going to take some organisation....