More power for what use? You use 80-100 HP cruising, have 200-300 HP for acceleration and climbing hills, the higher figure only if you are making the engine scream.
The best you might gain would be to raise that 300 HP peak to 330-360. The difference would be barely noticable. You seldom get to that power peak except for fractions of a second before upshift during full throttle acceleration.
You are probably accustomed to driving a 3500 pound car that needs 20-30 HP to go down the road but has 120-250 HP on tap for acceleration. To get that level of performance in your 14,000 pound Class C motorhome, you would have to raise output to 500-800 HP.
There are no easy fixes to get there. It has been done when the V-10 gets stuffed under the hood of a Mustang for drag racing, but you don't have room under the hood of an Econoline for the parts that get added, and you probably wouldn't like the fuel consumption, or the 5-200 hour engine life.