Unfortunately, it appears the sales rep was not clear on your wants/needs with respect to how you were planning to use the rig. I have found at numerous RV shows that reps (in general) do not know the OCCC of the rigs they are showing or do not care about it, i.e., "push the metal out the door mentality". In general, Class C rigs are limited by the weight they can safely carry, not space.
As noted by others, three options add the most weight to a Ford E450 chassis, Overall length of the RV (longer=heavier), slideouts, and automatic levelers.
Given your situation, understand that you did the right thing by weighing your rig, even though you are unhappy with your findings. At least now you can decide how to best minimize your weight for your trips. Others have provided excellent suggestions to start, like minimizing weight of fluids on board. Also, do you *need* to carry a grill with you? Can you cook over a camp fire? Avoid can goods. Carefully select clothing. If you are taking short trips, do you need to carry a spare tire and rim?
Good Luck