To be Frank, although my real name is Michael, I was once quite interested, but after a bit of thinking and research I wouldn't touch a DIY aluminium chassis with a ten foot pole.
For all the reasons stated so far. Aluminium does have a higher strength to weight ratio than steel but, given the density difference, the members have to be a
lot bigger.
As someone has stated here, by the time you match strength, the actual weight difference isn't as much as you might think.
Welds at home can be a problem. Need to get everything spot on if you don't want cracks down the road.
I checked out a commercial aluminium boat trailer the other day. All connections were made with galvanised steel brackets and stainless steel (I think) bolts.
That's how Quintrex, one of the country's biggest aluminium boat builders makes its factory trailers.
To do that at home requires real engineering, not "if it looks right, it probably is right".
Philip wrote: Nice welding. But I would weld the front of any connections that face into the direction of travel. The Y that ties into the tonque. Where it meets the cabin frame needs welded to the Y. He did side weld's but that does not keep wheel thrown trash packing that unwelded connection full of debris and rusting the tubes out.
Welding across the drawbar tubing runs the risk of warping the tubing. Better to run some seam sealer in there before painting.