The drawbar and axle are galvanised. The outside 20/25mm square section tube is not and will rust. This has been a serious problem for a comparatively small number of members. I'm unsure about other chassis members as they are usually covered in black underseal.
I would not buy a 20 year old van, or newer for that matter, without giving it a good look at.
If it s the Troll currently on Gumtree in Cornwall it is incorrectly advertised. It is a 530 and not a 540. It does look nice in the photos but I'd say the price is a bit top heavy. It is a sellers market though.
Colin