dlovitt......Read this link Diesel Performance chip 400 Cummins
it's the chip that your asking about.
On the last page, you'll see where one of the members had his dyno tested. It added 52 HP and over 100 Torque. On Monaco rear radiator coaches, they also offer an improved engine fan that gives the ISL an additional 25 Hp.
I have the chip and the fan on mine, but I didn't add it until it was out of warranty. Allegedly, the DIGI chip discussed in the link is invisible and will not allow the engine to exceed it's rated HP and Torque at wide open throttle. It adds power through the midrange.
I saw a legitimate .5 mpg increase with the engine fan, but I can't really tell what the chip is adding. It's improved my mpg, but definitely not an additional 2 mpg. Most are claiming an additional 1.1 - 1.2 mpg increase. I will be leaving on a 6000 mile trip next month and will get a true reading on my increase. The chip seems to have smoothed out my power band. When I hit a grade, I drop into 5th and can do at least 55 mph or better towing my Chevy Silverado, but I'm a short coach and weigh in at about 33K when towing.
I think the ISL is one of the best all around engines with a ton of power, but your coach is bigger and heavier than mine and may suffer a little more in the hills when towing.
Lastly, I didn't chip mine until it was out of warranty, plus the chip didn't come out until about a year ago. You didn't say what HP your ISL was, but some can be boosted from 400/425 to 450 by Cummins. They charge somewhere around $2500.00 for this, but they continue to warranty your engine as part of the price.
I wouldn't chip yours until you get some miles on it. It won't be huge, but your coach will pick up some power when it breaks in at around the 10K mark.