Running wire from 5th wheel hitch to basement area

I am guessing that you plan on putting genset in pick up box and want to connect to the converter.
Where is you converter located in the rig?

If converter is toward the rear of the rig. You will have to pull down the under belly material and rout the cable through the frame and along the frame longitudial rails.
You can use 10/2 with ground NM cable. Make sure it is protected around sharp edges.
Then connect to standard 30 amp cord in the front.
Might be a project!