I have a 36 Classic and the exact same project in mind…..
What layout have you got? Is the Heads at the front (3 cabin) or at the back (2 Cabin)?
If its at the back then the obvious place for the tank would be in the aft locker against the heads bulkhead. If at the front then I guess it may have to be under the forward bunk?
On the 36 Classic 2 Cabin, I have got a wardrobe in the Salon in front of the Heads, and have found that behind that wardrobe, there is about 120lts of dead space, which is where I am going to put my tank…..
When you are down at the boat the only advice I can give is to look in the back of all the closest cupboards, and see if there are any panels that unscrew to see behind… Thats how I found all that space I can fit a tank.