I believe Stripit is correct on the placement--you really need to get the unit weighed to see what your pin weight is.
If it was done correctly, I don't think it will add appreciably TO the pin, just won't take any off of it.
My '05 36" w/gen, w/d, had a PW of around 4400; current '15 38' has a PW around 4500. You should be ok with the new dually, but you do need to get all weights figured out.
Hope it turns out to be a good unit.
And, welcome to the forum.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired