Rustproofing caravan underbody

Don't know about the car ramps. but years ago, on a single axle 16 ft van, in the back yard, I jacked the front up until the rear touched the ground and with multiple jacks under the front for security and old carpet on the concrete for comfort, I brushed on Rust converter to the chassis, gave it a wipe over, brushed on the metal primer on the chassis, then I painted the under side of the van ( with mask, goggles and headgear.)

The 4 sides of the van were masked off for protection

For the rear underbody the towball /A frame was on the ground.

From memory it helped a bit if its windy so any paint fumes do not hang around and blows away from you. ( May have used fans as well). I my aim was to protect the wooden under side ply that was opening up ( small surface cracks).

POR15 is the best stuff you can get

Paint
On
Rust
POR
Not cheap but has a awesome reputation in the car repairs game

I know it is a more expensive way to buy it but using Penetrol in the aerosol cans is much easier to apply and it gets into all the nooks and crannies better too. Makes a crappy job a lot easier so worth the extra dollars IMO. If you have access to a spray gun you could head down that route as well

Since nobody else has said it, don't get under anything supported by jacks. Find something that can't compress/topple.

Re ( I now have a couple of quotes for sandblasting and painting )

Could be a waste of money. Had the windscreen pulled out of the Sprinter, rust was sanded/blasted out and repainted( By a professional Org). 3 years later rust reappeared. Had the screen pulled out again and with KBS DIY treatment no rust 5 years later.

Even with sand blasting I would still use a Rust converter. I do not know how mush rust you have on the chassis, but $1200 seems low for a good job, if its extensive.

If the ply is iffy this should be painted as well, How are the underfloor cables/hoses pipes being shielded from paint.

I checked the nature of the rust on the chassis and it appears to be surface only and is not flaking. It could therefore be a candidate for KBS. It only remains to find a workshop who will do this as at my age it is beyond me.
Considering possible damage to the plywood I am ruling out sandblasting.
Thanks for all the helpful contributions.
Adrian