4.5kg gas bottle on A-Frame

A 25mm long 6mm weld is engineered to take a tonne (1T) of weight
Its coming down to the rhs wall thickness and bracket design.

If the rhs was 3mm wall
it will be fine
all depends on the bracket setup to the rhs

I have two bottles mounted on the A frame of my CT but they are on two RHS welded to the A Frame - been there 9 years over all kinds of tracks and they haven't moved. In your case I'm puzzled as to why only one square tube is used and the bottle holder is fixed to that - this doesn't sound stable to me.

We have one 50x25 welded across the a- frame on our van, it originally held 2 8.5kg bottles for 7 yrs,last year I took the big bottles off and fitted 2 4.5 frames.to the rhs with roofing screws and carried 2 small bottles. Most of our travels have been on rough roads. Innamincka x2,Haddon cnr, Simpson desert via mt dare,Finke,etc.,the bottles never moved at all,

In 1999, we ordered a Jayco Poptop van. I asked for the drawbar to be made 150 mm shorter (to fit in our shed). They made it 200 mm shorter. No problems, says me and hooks the van up to take it home. Full lock turn in the sales yard, and CLANG! The bumper bar of my Range Rover hit the gas bottle, which was mounted on top of the R side drawbar beam (by a welded-on 50 x 50 angle iron lug). When I got home, I had to make up a cradle which now sits in between the drawbar angle, and the 9 kg gas bottle sits on it. I screwed the cradle to the drawbar with Tek screws roughly in the centre of the drawbar sidewall (not on the top flange, which is a stressed areas. The cradle is made from 20 x 20 x 2.5 mm RHS and neither it or the gas bottle have ever moved since fitting it there.