Will there be a significant voltage drop over 60 FT if a 6-3 extension cord is used to power a motor home with house current?

no but you cant exceed the wattage of the generator if you are close to that amount you will notice a reduction not nescessarily in voltage but amperage which will have the same affect If the generator is 7500 watts, 240 volts, that is approximately 30 amps maximum load. According to one online calculator for voltage drop, your AWG 8 wire run would have only a 3 percent drop over 95 feet or so, or 120 feet if you're only using 25 amps. Ref: www.elec-toolbox.com/calculators/voltdrop.htm