Sounds familiar.
My 1973 G-26 has a similar set-up - outboard with the gas tank under the starboard rear cockpit locker. Exception - my batteries are located in cabin lockers.When I first got the boat 7 years ago, and before any major sailing, I called the coast guard in to do a total inspection. They found everything in order, with the exception of no ventilation for the gas tank in the locker. I was advised that I needed a ventilation port cut into the side of the locker where the gas tank was. The vent was to be level with the floor in the tank storage area so any fumes that were present would fall through the vent and into the cockpit sole and then out the open rear scuppers (level with cockpit sole). Was easy to do - picked up a vent cover at the chandlery, cut the opening, fastened the cover and Bob's your uncle. You will be pleased with this set-up, and so will your insurance company.Makes sense eh? Don't know if you were aware of it or not, but Grampian built many O'Day sailboats back in the '70s'.