I tow a Chevy Colorado with my E450. It weighs about 4K with the junk in the back that I take, maybe more. It pulls fine on grades up to about 6% which covers the freeways. It will slow down to 45-50. It might do more but the high revving V10 sounds like it will explode so I hold back a bit with the foot. The main problem going up the mountains is trucks. You can go much faster than they can, but too slow to get out in the passing lane without obstructing everyone. So sometimes you get stuck going 20 mph behind a semi for a few miles.
I'm not a Ford fan, but I have to admit that every time I drive the motorhome I am impressed by that V10 powertrain when I put my foot into it.