Not exactly true. It's not hard to learn to weld aluminium, but if you're considering doing structural work like the trailer beams, I'd leave that to the pros. Not much different than structural steel welding in that if it's a critical weld, a good looking weld can be weak, have inclusions, voids, lack penetration (cold weld), etc. and come apart on you on the road.
When I've done those welds, I've had one of the guys in our shop (AISC shop) do the welding. Something that can be overdesigned or have an overspecified weld, I'll do myself. Like repairs to the goal posts, or bracketry for bunks. But if a full-penetration beam splice was needed, where the trailer frame would break in half on the road if a crack started, I'd have a pro do it. I know my limitations, especially since I'm surrounded by those without those limitations.
Aluminium has the added bear trap of no fatigue limit, which means that unlike steel, there is no stress level below which fatigue will never occur. Welding increases the stress at the weld interface (stress concentration or stress raiser) and changes the metal in the area outside the weld (heat affected zone, or HAZ), so you have to design for that if it's something that will experience repeated stress. Like a trailer.
But AFAIK, the Mac trailer as well as most other aluminium trailers don't have many (if any) welded features on the beams, and simply bolt the axles, tongue, and other structural parts onto them.
