The "E" indicates the load carrying capacity of the tire, but the sidewall should also state the maximum it can carry in lbs. You determine the right tire by either max axle capacity (GAWR rating for the axle) or by actually weighing the rig, front and rear axles separately. You want tires that can carry the max load (just in case), but you can adjust the air pressure down from the max shown on the sidewall if you customarily are loaded substantially less than the max.
An "E" rating is an approximate load range - one manufacturer's "E" may carry more or less than another's, but they will be fairly close.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL