We have a 2003 ML 350. Can this vehicle be towed behind our RV with "4 wheels on the ground" ?

No. from your owners manualhttp://www.mbusa.com/vcm/MB/DigitalAssets/pdfmb/ownersmanual/2003_ml320_ml350_ml550_ml63.pdf Page 499; 499 Practical hints Towing the vehicle  Towing the vehicle Mercedes-Benz recommends that the vehicle be transported with all wheels off the ground using flatbed or appropriate wheel lift/dolly equipment. This method is preferable to other types of towing. If circumstances do not permit the recom- mended towing methods, the vehicle may be towed with all wheels on the ground only so far as necessary to have the vehicle moved to a safe location where the recom- mended towing methods can be employed. When towing the vehicle with all wheels on the ground, the vehicle may be towed only for distances up to 30 miles (50 km) and at a speed not to exceed 30 mph (50 km/h)

get a car dolly from Uhaul. theyre pretty cheap and they install