If you are renting a mobile home and after 6 months of living here, they without your knowledge come and take the tank out for fuel to heat the home and never tell you nor give you any reason nor replace it, is this legal?

Who took the tank? Did you ask the landlord about this?

If heat or hot water was part of the rental, then yes, it is illegal. The next question is what do you want to do about it. Do you have a lease? ANY question on law needs to include your general locale as laws vary by area.

I think your first point of call is the landlord.

Ok, tank for the fuel. I will have to guess this is either fuel oil or propane.

I can't see a fuel oil tank being moved ? But it is common to move the propane tanks. They are normally owned by the propane company and filled by the renters.

Are you paying your own cost of electric and gas ?
Is the heat suppose to be included in the rent ?

If you have to buy your own gas, then yes it would be normal for them to take the old tank out, ( most likely was rented by the last tennant)

You need to discuss with this with the landlord.