How to open fresh water tank

Wanting to clean our fresh water tank I have attempted to open the round inspection hatch but it is completely stuck. I have tried turning it in its counter clockwise direction with hands, wrenches, hammer + wood, and applied heat with a hair dryer. Did anyone experience difficulties opening the tank and what did you do?

Yes but there is a special hose that runs from the tank to a hole on the side of the boat that should take care of that if I'm not mistaken. Btw. it's 1/4 full.

I would manufacture a special tool for removing the lid. Use a solid piece of wood 50-70 cm long, and place two pieces of ½" pipe secured with two bolts to the wood. This way you will get the torque you need. Hammering on plastic in cold weather can cause unwanted results.

Thank you for the replies. I tried opening the outside water tank inlet to eliminate any vacuum but I was still unable to open the inspection hatch. I'll try to construct a tool based on your advice and try again.

I just use a pair of pliers or pincers.  Put the ends of the handles into the two slots opposite one another and the just put a piece of bar or wood through the handles so you can get more power.  (you are using the pliers upside down.)