The things you are describing aren't going to make much difference. Adding timing on a heavy vehicle is just going to cause spark knock and the computer is going to pull that timing back out. From what I've read about these engines, enrichening the fuel mixture actually decreases output (the sweet spot appears to be around 13-13.2:1, whereas older engines and induction systems might make more power at 12.5:1 than at 13:1). Those are the way "chips" increase power on fuel injected engines. There is no chip in any current generation vehicle and what you're really talking about is called a "tune," where a technician uses a laptop and software to modify the contents of the ECM and the parameters that affect engine performance.
If you really want more power, a supercharger is probably the only realistic option. It costs about $7,000 for the parts. If you do it, you should be very modest with boost but you could see another 100 horsepower even with modest boost numbers.
Changing heads, a modest camshaft change, etc., could produce noticeable gains, but you better be prepared to spend $2,500-3,000 for them and be VERY careful in your camshaft/compression ratio/head port selection or your motorhome will be sluggish, especially around town for a gain of 1/3 to 1/2 of what the supercharger will produce without any other modifications other than a tune, which both will require.
Keep in mind that the horsepower your motorhome has is double what one would have 30 years ago for the same size motorhome. Maybe you don't need that power after all.
