I take it there are already springs in the arms, otherwise you wouldn't be wanting to replace them. Take the arms off, one at a time, and straighten them. Undo the cable and spring from both ends and pull them out. Get a stronger spring, or a new cable as required and fit the new bits. Repeat for the other three arms.
I had a camper many years ago which had not lift assist at all. The local sailing shop made up the cables and a hardware shop had suitable springs. Getting the geometry right was a bit tricky but since you've already got an old system in place that part of the problem has already been solved.
If by chance some previous owner has removed the springs and cables then it's a bit more difficult, but looking at another camper should show you where the pulley has to go, and the rest is logical.
Would anyone know how to replace the spring & cable inside the lift arms on my little camper?
Its just to heavy to lift up with no spring in the arms
Hi Aussie Camper
A suggestion might be to look at the older style garage doors which had a spring on the lifting arms to assist the door lift. They had a flat metal door which lifted and swung up flat into the ceiling. The long springs had an adjustment with a cable or chain to get the balance right.