Trying to actually see them (opposite of what most issues related to this are) but for some reason the config just isn't there. PrefilledGasTanks is however and enabling that shows the gas fine.
Forge: 14.23.5.2808
Mekanism: Latest

Edit: Adding the options manually in the config seems to be registered in Mod Options ingame, but it doesnt actually change anything

There is indeed no option to add prefilled tanks of liquid (possibly related to the portable tank being on the same item as other machines)

Edit: Adding the options manually in the config seems to be registered in Mod Options ingame, but it doesnt actually change anything

Yeah it's based on forge's system which will show whatever config options it reads from the file.

This request has been added to the master list at #5322

In order to keep issue numbers under control this issue will be closed and used as future reference.