my coachman, has a black plastic sheeting stapled to the underside of the wood, factory fitted, under neath that is bare wooden ply floor, had to replace a small piece, that had tore off, a while back, just replaced with some heavy duty black plastic rubble sack.
Eriba touring is a different proposition. They are built differently from other caravans. What you are talking about is the steel tube framework on the underneath. This does need painting to keep the rust away. If rust seriously gets into an Eriba Touring the caravan it can be a write off beyond economic repair. When I had an Eriba Pan I painted the steel framework & underside with black Hammerite.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.