The V-10 is a robust, nearly bulletproof workhorse that you will not hurt by letting it run. Do your homework folks, if 4,000+ rpm offends then the V -10 is not the engine for you. Max torque at 3,200 and max Hp around 4200-4400 is where this engine does what it does best.
Put your foot into it and go. Please do not crawl up the long grades at 25 mph to save a couple of dollars which besides being stupid, dangerous, rude and illegal in most states (hello, there are minimum speed limits that apply), it is usually unnecessary. If you cannot climb a 6% grade at 50-55 get thee to a scale and you might just find out why. If you are within your weight limits the only weak limit is the driver. Keep in mind that 305/420 V-10 in your E-350 is also found in most Class C's up to 32' and fair number of Class A's.
Yes a light right foot garners better mileage, I cruise at 55 to 62 max but the power is there at your disposal as needed and long grades are the time to tap in.
As always.... opinions and YMMV.
:C