Electric cord extension advice...........

Is your rig 30 or 50 ampere?
Rigs normally come with #10 wire for 30 amp and #6 for 50 amp.
Depends on what you are trying to operate.
30 amp with one AC unit, no water heater 120 volt element, you should be able to utilize the #10.
50 amp running two AC units, water heater on LP, you should be able to use the #6.
Building extension cords gets expensive when you start buying the connectors.
30 amp extension cords are cheaper off the shelf at Walmart.
50 amp are expensive no matter which way you go. Easy solution is to reduce load and use a 30 amp extension cord with dogfbone adpater.

Doubt if you can make one cheaper than you can pick one up at WalMart. I just paid $1.40 a foot for 12guage plus ends to make a short cord for another project. $27.00 with ends for 12ft. You need 10/3 which will be more plus the ends. Better to buy a premade with nice molded ends.

Look at your cord it should state what gauge wire you have. I would go one size larger. Price it out by the time you buy wire and fittings you may nor be saving very much. If you build it yourself make sure you use double insulated cord.