Your engine computer will prevent you from over-reving the engine. 4,000 RPM isn't even in the peak power band of your engine, according to the year of the engine, it's closer to 4,750 RPM for peak torque/HP. The V-10 engine is made for high revs, it's got smaller pistons but more of them to give higher RPMs safely. Running it below 4,000 RPM climbing a grade is actually harder on it than letting it rev higher.
Check the information sticker next to the driver's position. It will give you the GCWR(Gross Combination Weight Rating) which is the maximum weight of your RV and toad. The maximum weight of your RV alone is called the GVWR(Gross Vehicle Weight Rating) The difference between the GCWR - GVWR is the maximum weight of any towed trailer or auto. If you actually take your RV to a truck scale loaded for a trip, then you could subtract that # from the GCWR and see what's left for the toad.
There's many options and opinions on towing, 4 down or with a dolly. 4 down is convenient, but costs a lot to set up unless you keep an eye on Craig's List for used equipment. Using a dolly allows you to trade cars more easily but then you have storage and strap down issues.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI