No idea, but I've never seen a non HD payload F150 with more than #4050 rear axle rating..
Even on #7200 rigs, they seem to have #4050 rear axles.
My #7700 Max Tow only has a #4050 rear axle.
When I weighed it last time with my loaded #5000+ TT in tow I was at #3400 on the front, #3640 on the rear and #4500 on the trailer axles.
I ran my 97 F150 over it's GVWR and rear GAWR for over 11 years towing the same TT. It had the same Sterling 9.75" rear axle as the 13 F150 and I was running LT rated 16" tires and overload springs on the 97.. Yes, I know either of those did not raise the weight limits, but I was okay with it.. Or just got lucky.. ;)
Dunno how you all are staying under your #4050 axle weights with the trailers you are towing with your Eco's, but as long as you are comfortable with it and it's working for you, I've been there too. ;)
Mitch