Good Engine, easy to maintain, air filter size is small so it clogs easily. Keep a spare. Oil changes are easy, has variable cam timing so change often such as at 5000 miles to keep the variable device from silting up.
We have a Forest River 2551TS which weighs 13,600 loaded and gets 9.5 to 9.7 mpg at 65 mph. Plenty of power, crests Eisenhower Tunnel at 65 mph. When new it would only do 45 at the last stretch, likely a Ford break-in de-powering mode. The engine is suppost to be limited to 3900 rpm; however, it will run up past 4500 at full throttle. The interior engine cover stays cool.
The E450 chassis like all chassis needs alignment after Forest River adds 7000# to the partial chassis. For us this was a 1-1/4 degree camber/caster bushing (passenger side) installed all toward caster and 0" of toe-in. This chassis hates toe-in, as it caused poor steering response and a measurable reduction in fuel mileage.