The Fluid wagon research does not appear in the tech tree. Disabling all other mods yields the same result so no conflict with something else either.

It seems that the Fluid Wagon and it's research are disabled by default in the mod's startup settings. I had to manually enable it and restart the game.

Mod author may have disabled the tech by default since it's probably not really needed tbh.

"adds fluid wagon MK2/MK3(standard disabled, can be activated via settings)"
That's right. I have disabled it by default because I think you do not need the improved fluid wagon.