Caravan build 18ft

Hi

I’m starting my second build (first was just an enclosed trailer with 3mm composite cladding)
This time around will be a caravan, would like to hear from others that have self built on what frame method they used and how they found it?

I have access to the below
aluminium channel, tube, etc - but can’t alloy weld
Steel rhs, shs - good supply, reasonable price - can weld this well.
Timber meranti - I work for a supplier of timber and should be able to get in quantity for reasonable price.

My thoughts at the moment is SHS/RHS frame in 20x20 1.6 or a mix of 25x50 and 25x25 in 1.6. With 500mm centres, 4mm composite cladding with foam insulation. 3.7mm caravan ply internal.

I’ve considered the weight and steel will be more but still under my target TARE

Done right, steel should have less impact if water leaked?

Should I seriously be considering timber frame over steel? Or aluminium given I can only rivet together?

I’m not considering the full composite panels as I am too far away from suppliers, they won’t ship? And I may make internal changes on the fly.. and cost would be about triple?

Any general thoughts on caravan frame?

Thanks
Jason