Since you posted this on two forums and there are some misconceptions here, I'll post my response again.
When Monaco went out of business, a couple of guys created Source Engineering and began making products to improve some of the issues with Monaco coaches. As an example, they made new trailering arms for the Roadmaster R4R chassis that had issues. The fan was originally designed for the Roadmaster chassis.
The Monaco rear radiator coaches, were/are built differently than most other brands. Monaco took the charge air cooler and placed it on top of the radiator, while Freightliner stacks theirs one right in front of the other. The Monaco setup cooled more efficiently and was less likely to get dirt built up between the units, like the Freightliner's do.
On my 2005 Monaco Diplomat, I changed out the radiator fan to the Source Engineering fan. It didn't provide less air, it was just better designed. The stock Monaco fan was pretty rudimentary looking, while the Source fan was very well engineered. Since the fan performs better, it doesn't add horsepower, but gives back horsepower taken away by the other fan. The amount of HP returned is determined by the engine. The 400 ISL got back 25 HP because it was a slower spinning motor. The ISC got back almost 40 HP because it spins faster.
The new fan could be felt in acceleration. We live out west and I was routinely towing a 6K truck. The OEM fan and the Source fan performed IDENTICALLY in the heat and while climbing in regards to cooling. The new fan was also quieter inside the coach and at the rear of the coach.
When I replaced mine I did some investigation and looked at the Horton fan clutch and the Source Engineering fan. The Horton fan saves power when the engine is running cool on flat ground. As soon as you start to climb, the Horton unit engages and nothing is saved. The Source fan is saving power while on flat land and while climbing. With the Source fan, I saw a .5 mpg increase.
The fans are pricey, but well worth the performance and mpg increase. On my Diplomat, I was able to remove an interior engine cover (floor panel) and install the new fan in about ten minutes with an air gun.
Don & Mary
2019 Newmar Dutch Star 4018 - All Electric
2019 Ford Raptor Crew Cab