Vipermanden: If you already have a 50amp receptacle that you can use to power your RV from your house and it looks like this one:
http://www.homedepot.com/p/GE-50-Amp...054P/100193650 Then you are good to go from the house end. This is used in many applications as in a modern 50amp stove receptacle. When you are wiring for 50amp service to your RV you do have 230v going to the RV via 2 x 115v legs so don't be concerned about that.
Now to extend your service to the 75' you want, why not just buy a 50' x 50amp RV extension cord like this one:
http://www.homedepot.com/b/Electrica...s/N-5yc1vZc4ny For under $165.00 you can have what you want, and I do not believe you can get one made by an electrician for that price.
If you start from your house with the extension and use your supplied cord for the RV you will not have to worry about the end that fits into the RV either...win, win...
Good luck