Hi,
While filling the fresh water tank is important (but not critical) to loading up the trailer to set up the WD system, loading the truck is not important at all.  The angle of your truck bumper will not change at all with ATV in or out.  
Depending on what type of Silverado you have - 2500 or 3500, then the WD bars will not need to be all that tight.  1500 and you need them pretty tight to apply some weight to the front axle. 
If you have a 1500 series truck, you probably should take it to a scale.  2500 and you probably have over 2,000 pounds of cargo rating, and are not so likely to exceed your rating.  With a dually 3500 series, then the scale will tell you how much you can still load up the truck before it becomes overloaded, more a FYI than any needed item.
I would not take the ATV when going to the scale.  If you are 'close' you can add all those extra weights in later.  You don't want to take all that extra weight until you know that you are at least 'safe'.  IF you have a 3500 truck, then you will find out that your rear axle is only 4,500 pounds and you still can load 2,400 pounds and not exceed the tire rating, the ATV weight will not effect driveability or safety.  If you have a 1500, then tanking the ATV might mean you are overloaded and you will have no idea by how much. . .
Fred.