You are correct Wmtire. Based on what I have been able to find, he could have up to a 100' cable with #6 wires.
My son, having read/heard of cables overheating, logically or illogically would like to have the shortest run possible.
For his peace of mind and since the 15' cable is about 12 years old, a $37 cable seems fairly inexpensive.
patrickscott, if I am following you correctly and from the link you posted previously....... all you are doing is replacing the RV's 15 foot shore power cord with a 18 inch shore power cord. The connections are the same.
Now you are going to plug the 75 foot extension cord into the 18 inch shore power cord instead of a 15 foot shore power cord.
You are still going to have two cords (the shore power cord connected to the RV + the 75 feet extension cord going all the way to the power pedestal) no matter how you do it.
The only difference is, that with the 18 inch shore power cord, you will have 13.5 feet less of total cord length (75 feet + 18 inch = 76 feet six inches)
as opposed to 15 foot shore power cord + 75 feet extension cord = 90 feet total.
Seems like a lot of trouble and expense for 13.5 feet less.