Sounds like you have gotten your phantom loads as low as you can. You just have to live with the rest. Though you might invest in a LP detector that runs on batteries.
The only other thing I can think of it to shut the water heater off until you need it. Even then , it might draw a little power. You would have to check it. Problem is, I suppose that this might use more propane. Depends on how often you turn it on and off and how well it is insulated.
I have done the same as you but, I do not have a LP detector and my water heater is just a pilot light model that uses no power.
I can go 2-3 days easily without falling below 50-60 percent on my 2-6 volt golf cart batteries.