You have a lot of variables that change from unit to unit and for the different ways they get loaded.
For example revfly, your fulcrum point should be your middle axle. For the 1800 lbs that are loaded in the rear you would need to find the center of gravity point for you load. Then the distance of this CoG to the center axle. Then you say you've went a filled the water tank to 1/3, now you need to figure the CoG for this load, if it's in front of the center axle you've added back some of balancing factor of the rear load.
Now of course I'm sure you loaded the basement, add all that CoG load back on. Everyone has different loads and loads them differently. The only way to be sure is to go to the scale. If you just want to figure the garage load effect on pin weight you'll have to unload everything else. Looks like your going to have to give up those 2 lb T-bones and eat 1/4 burgers.