alco suspension

They can. Al-ko's have a very narrow window of optimum weight ratings, but then again so do leaf springs.
The biggest disadvantage with coils is that running shockers is a must and can therefore be a bit on the pricey side when comes time
to replace them. They are also bulky. Not many trailers, campers or vans have huge wheel arches to accomodate the length of
a coil and shock absorber. Don't get me wrong, coils are probably the softest riding suspension with the longest travel,
but other than multi-axle, I don't know of many trailers that need wheel travel. Having worked in the industry for a number of years,
including Al-ko, everyone has their own opinion as to what's "the best". In my opinion, the best suspension set-up for a single axle trailer
is the "knee-suspension" produced by Melbourne Trailer and Caravan Supplies. A simple leaf spring set-up that does not use the springs to
hold the stub axle captive, so if you break a spring, it won't tear the axle from under the trailer. And there is no axle as such,
it uses a wish bone type that pivots from the middle of the chassis, so your ground clearance is the maximum available.

I understand your concern with the wheel alignment of the "knee" suspension, but as with any independent suspension, the wheel alignment
is set up to be correct at the load rating that the particular vehicle is set up for. Even solid beam axles can suffer this to a certain extent.
Try putting 1500kg on your 6x4 trailer axle and see how much you'll wear tyres. Irrespective of your suspension choice,
make sure you know the load of the towed vehicle, including all the luggage you put in it, and add a safety margin. That way
you shouldn't have to worry about how easy the trailer suspension is to repair in the middle of nowhere! Happy Towing.