My '11 Sunseeker 2300 has a proper low point drain valve, just ahead of the water pump accessed via the access door in that storage compartment behind the right rear wheels. The hot water drain is there too, although that requires getting underneath and unscrewing a plastic cap on the pex. Both drain lines are routed through the floor right next to the other, blue for the cold and red for hot.
Also, be careful about that water filter housing. Mine sheared off at the threads, fortunately just with a load of pink stuff in it and not with a full water tank. I just spliced in a piece of braided faucet plumbing line as I discovered it during an on-the-fly day-of-departure dewinterizing as I was rolling out on a mid-winter trip to Florida in January of '15.
And speaking of checking on exactly how full the water tank is, mine has a a screw plug access underneath the corner mattress, the intended use of the hole to turn the hot water heater bypass valves, but it also works for shining a flashlight in to check the volume in the clear tank, located adjacent to the hot water heater under the bed.
