I have a converted Hino bus in the workshop at the moment that the original owner home built and wired everything using cut up extension cords ..... then fitted the floor and the rest of the fitout
There are length of extension cord with 1 mm sq cabling running to each power point and a length joined somewhere between the 15 amp inlet socket and the 240vac power board, it runs from one end of the bus to the other yet the safety switch is at the power box end .... so this dodgie cabling is completely unprotected.
The minimum size conductor for power cabling is 2.5mm sq and 1.5 mm sq for 240vac lighting. They don't make extension cords with 2.5mm sq cabling unless it is an extremely long cord and generally restricted to industrial site tradie use .... mostly because it is so expensive.
That "plug and play" cabling Jayco and a few others use is 2.5mm sq cable, if you ever see a bit of it, you will recognise just how thick it is .....
For those that are asking why a 15 amp extension cord is only 1.5mm sq, it is because the cable is out in the open so it gets plenty of cooling, put it through a cupboard or inside conduit and the temperature capacity rating is immediately reduced, run 15 amps through 1.5mm sq cable inside conduit for any distance and the risk is the cable will heat up so much the insulation around the actual conductor wire will melt and possibly catch fire ....
T1 Terry