Suggestions
Your surveyor is correct. I had heard the "theory" of the gas fumes sinking, and the possible consequences, but basically ignored them on my O'Day 25. I kept the gas tank in the same place as you described. Then, my wife and I took a trip down the Texas ICW, and needed to bring extra gas. I stored them under the cockpit as well. About halfway through the first day, I went to get an extra tank of gas, and couldn't believe how strong the fumes were. That's where the battery is as well, so out came everything. I strapped the extra tanks on to the bow, and put the regular tank in the cockpit, and it's been there since.Part of the problem with trying to keep it under the cockpit is that you'll have to figure out some way to get it high, and enclosed, with a through-hull venting the fumes overboard.The link below shows an elegant solution, still keeping the tank in the cockpit. I'm going to do this on my boat, but with starboard instead of wood.