Using the GVWR of 7,500lbs as a worse case loaded weight scenario, a 13% to 15% desired loaded tongue weight range would 975lbs to 1,125lbs....., so a WDH rated at 1,200lbs would be required.
Also, a 31ft loaded TT will tow much better in the 13% to 15% range, especially in less then desirable towing conditions.
Modest 27DSRL example: (5,400lb UVW) + (450lb Ship Weight delta) + (100lb RV Dealer, battery, etc) + (600lbs cargo, no fluids) = 6,550lbs
So your targeted 6,800lb loaded weight is realistic: (13% to 15%) = 884lbs to 1,020lbs, so a 1,000lb rated WDH would be fine. However, if one tends to load a little on the heavy side (family, fresh water, etc.), then I would stay with the 1,200lb rated WDH. A properly adjusted 1,200lb WDH would not be overkill with a 900lb plus tongue weight.
Departing the RV Dealer you were in an "unloaded" TV/TT condition, so your WDH would have to be re-adjusted anyway under TV/TT loaded conditions.
To insure that your WDH is adjusted correctly, a CAT scale visit under loaded conditions will tell you everything you need to know.
CAT Scale 'how to': http://www.jaycoowners.com/forums/f37/how-to-weigh-your-tv-tt-3871.html
Hope this helps with your decision.