Well, if your limit is 5000/500 without a WDH, and 10,000/1,000 with a WDH, then YES, your tow limit just doubled by adding a WDH! I'm just saying that the reason for the doubling is not because of an increase in pulling ability (5000 vs 10,000). It's a structural/stability issue (500 vs 1000).
But you can't decouple the trailer weight from the TW, because loaded TW should always be a minimum of 10% the total weight of the trailer. You would not want to tow an 8000 lbs camper with only 450 lbs of TW, and justify it because you are under 500 lbs TW.
The problem with the dealer is that they think you can pull the full MAX Tow Rating without a WDH (wrong!) and that a WDH will allow you to tow beyond the stated MAX Tow Rating (wrong!).
thebrakeman ('70), DW ('71), DD ('99), DD ('01), DD ('05)
2004 Surveyor SV261T (UltraLite Bunkhouse Hybrid)
2006 Mercury Mountaineer V8 AWD Premier
Equal-i-zer WDH (10k), Prodigy Brake Controller