James, our boat is a1990, Mk 1.5. We have 3 water tanks, including one under the v-berth.  I'm pretty sure that it was a factory option, so it's completely feasible. 

James, we also have 3 tanks, one in the bow, on a 2002 MKII.

Check out this link from Catalina Direct for more info.

https://www.catalinadirect.com/shop-by-boat/catalina-38/interior/plumbin...

This is an old post so not sure if it's still relevant.

My 1987 C36 has a 20 Gal tank under the V berth which required the sacrifice of two of the three drawers in the forecabin. Not sure if it was a factory option or fitted by a PO. Very professionally done I have to say. However when full of water the bow was very low on the water line and looked unwieldy. With the tank empty it sits much better in the water. I have 180' chain, 180' rode and 30lb anchor. Long story short, I'm about to fit a new 15 gal bladder tank under the aft cabin double berth on the port side just aft of the aqua muffler next to the main 25 gal plastic tank. I'm also fitting an 18" sq inspection hatch to aid access for installation and maintenance. A 1.5" deck filler sits forward of the main tank filler on the port side deck. I also have a 5 gal water container in the aft lazaret, so that takes me back to the 20 gal capacity of the forward tank. I simulated the weight distribution of the new tank using several water filled containers and it does not upset the port-stbd balance.

I'm removing the forward tank this winter and will rebuild the lower two drawers.