How do I go about checking the gas tank? is my only option to disassemble the whole thing? As I mentioned, there are no visible leaks. On my previous Maxima there was a fuel leak on the fuel rails connecting hose and it was very visible. So I'm puzzled how can it smell that strong and be invisible?

In all likelihood your tank is fine. But to access it you remove the back seat and there is a round hatch underneath. You can access fuel pump and gas lines through the hole.

The only source of gas smell under the hood can be the path of gas from the line, through the fuel filter, into the rails and to the injectors. Everything past the injectors is combusted, so no further source of gas smell.

Did you make sure injectors are not leaking? Fuel filter clamped well? No misfires and exhaust leaks at manifolds, Y pipe and cat?

I'm sure that there are no exhaust leaks. And there are no visible leaks on the lines that I can see from fuel tank, under the car, under the hood to the fuel filter and then to the engine. I'll try to investigate it under fuel pressure as now it does smell after a week or so of standing. If I drive it daily it doesn't smell. So it's some dry cracked hose somewhere.

Maybe a fuel vent or evap line! Remove the Back seat and inspect your rubber line condition! Inspect the entire fuel/evap/fuel vent lines for possible cracks or deteriorating lines or connections! Engine bay/under chassis/