Road Kings come with 12" rear shocks. Electra Glides come with 13" rear shocks. So a Road King is approximately 1" lower at the rear than the standard Electra Glide models. It's been that way for a long time, so I'm not sure why this dealership isn't aware. Your Road King has the same main frame, the same rear frame, and the same front suspension as the Electra Glide models, including the CVO Ultra. The only difference is the rear shocks, and if that dealer would like to look up the part numbers he will find that the Road King you have comes with the low profile hand adjustable shocks, which results in a lowered seat height of about 0.75" compared to a bike with the shocks that come on the Electra Glide. Perhaps you just need to find an independent shop to install your front lowering kit if the Harley shop can't figure this out.
If you just want to lower the bike to more easily get your feet on the ground, I would not recommend going with an even shorter rear shock. You will give up too much in terms of ride quality and clearance. Look into a re-contoured or custom seat with a lowered rider "pocket" that is narrower at the nose, and some good riding boots with thick soles and heels. Many folks have taken that advice over the years, and those who have reported back seem to agree it's a better solution than running an ultra low rear suspension.
Jerry