Right now, none, as they don't meet US emissions standards. I am guessing that the demand is simply not enough for the engine makers (Cat and Cummins) to invest in testing and certification.
The Signature 600 Cummins and Cat C16 are only useful to specialized needs such as heavy haulers, and much of their appeal for that market is the torque rating of 2050 foot pounds. That rating also takes one hell of a transmission. The Signature 600 is still available in Australia, where they run heavy combinations (wagon trains); I've seen it on the Australian Mack Trucks web site.
There are Cummins ISX, Cat C15, and Detroit Diesel (14L) motors which are rated up to 575 HP and 1850 foot pounds. These would be more than ample for any conceivable RV use.
One of the reasons you are less likely to see these higher ratings in the typical RV market is the transmissions. That market is concentrated in the typical dullard driver, who is not at all interested in driving performance, etc, and cannot and will not be bothered with anything but the easiest operating conditions. My recollection is that the typical (non-truck) automatic transmission in those units is maxed out at about 1650 foot pounds, which is actually plenty for the purpose - these are not heavy units, typically only about half the weight of a typical road semi.
My preference is for a Roadranger 13 speed; I think it is the most flexible and useful option for the majority of conditions. However, the co-driver issue for me would preclude that, leaving me with the autoshift 10s and 18s as the choice. Given the (comparatively) light weight of conversion RVs, I think the autoshift 10 will do virtually anything needed, and is probably the best return on investment. (In a semi, I think I would go with the 18.)
Each engine manufacturer has its adherents. The ISX 500ST looks like a heck of a good motor, with 500 HP, 1650 foot pounds, running up to 1850 in the top 2 gears. Honestly, though, that's more than needed; I would seriously consider specing that with an 18 autoshift in a semi if I had to go back to that life.
Likewise the corresponding C15 Cat, which seems to have taken a small beating from the emissions issue - the majority of their multi-torque ratings are gone. The advantage of the Cat to me is that in the C15, one gets not only a Jake, but their proprietary hydraulic retarder for additional braking power without using the service brakes. That alone might overcome the additional cost issue for me, since I am not likely to need the power.
In all honesty, though, a 11-12 liter motor with a rating of 400 HP or so, with 1450 foot pounds or so and a decent transmission, will do all one needs in this setting - other than the most extreme grades, I predict that there is little likelihood of a real drop in speed. The reduced load means that even this small engine is loafing most of the time, and longevity is not likely to be an issue unless you get a lemon. Fuel mileage will be better, etc.
You may be talking $10K or more increase in initial price, plus 1 or so MPG lost, to go the large power. It has real appeal, but little real value in an RV. Given the scarcity of real grades in most people's driving, and the fact that even those living in the most mountainous areas will spend little time actually needing the power, the big HP is probably not worth the additional cost and heartburn.
