I level along the top of the fridge
door. It's mounted to the floor and is manufactured square so the top edge of the fridge
is parallel to the floor. Got into that habit with a propane fridge
which has to be level to function properly.
Prior advice about not using heavy weight
in the rear to reduce tongue weight
is pretty good advice. The people who get all tech about it can explain it better but it boils down to sway is made worse by large amounts of weight placed further from the axle
in both directions.
On the other hand you should be able to tow with water in the tanks, might need to shift load around inside. As suggested possibly take a bin stored under the rear table and move it forward to over the axle
area when towing with water.
Do you by any chance have dual batteries or dual propane
tanks on the tongue?