Hey, came across this a little while ago and wanted to chime in, but it's been a rather long while since I posted anything here and I couldn't manage to get into my old account anymore, so now here's a new start. So anyway, I've looked into these a lot, and then finally found one with most of the specs I wanted and bought it, it's an 1120 AF. Based in Europe though so probably different perspective/circumstances.
First maybe something regarding those 2 letter codes, so there's a distinction between AF and AK (not quite different models but more specs/use related), wherein the F in AF literally stands for fire brigade, and AK tends to be utility/military. AF ones have a lighter frame. Then LK and SK are groups of light and heavy class models respectively, with different engines. From what I saw most people that have commented here with experience (as well as myself) own an LK/LN2, which have different versions of the OM366, which from what I've gathered is a great engine.
I haven't driven mine a whole lot yet, but from what I've read others with these are able to get around 20L/100km, I'm sure weight/shape/gearing makes a difference. SK models have larger engines and will be a little more thirsty.
Axle ratio and gearbox are important in general. I test drove 3 different LN2s with varying results. The one I own has the longest possible standard axle ratio (43:10), 6 forward gears, and ~200hp. I haven't done the conversion to larger size singles yet, but I imagine the resulting longer final ratio will do just fine in this case. Both others I drove were 917s, so same engine but ~170hp output. One of them similarly had a 43:10 axle ratio, but only 5 forward gears, and about the largest tires that would fit. It was too much, as in too little power with gear jumps that were too long. The other 917 had a shorter axle ratio and probably smaller tires, I don't remember whether it had a 5 or 6spd gearbox but either way it was fine.
In Germany these trucks (LN2) have also been called "Limo Laster", as in limousine truck, referring to their ease/comfort of driving compared to other similarly aged trucks. I haven't had mine that long yet so it still has most of the fire brigade equipment and old leaf springs, and I don't know how well the springs/shocks match the now empty almost 3000L water tank. It results in an actually quite wobbly and uncomfortable drive. However, the others I tested were in active use and both had parabolic springs, they were both pretty easy and smooth to drive (for someone used to older diesels). One of them was sound insulated and it made a big difference.
When buying this I had the intention to travel out of Europe to places with worse roads but where the basic technology would still be relatively easy to fix, I also like driving off-road (without looking for extremes). Compared to other trucks from before more electronics/common rail were introduced that would be suitable for something similar, Mercedes has a unique advantage regarding familiarity and parts availability, as well as available documentation and access to online information. I've been a bit overwhelmed learning about and dealing with the truck, but wouldn't say I regret it and still believe it to be a good match for the plans I had. On a sidenote here, I would potentially consider selling at the moment, change in life circumstances, send me a msg if interested. Anyway, for someone who just wants a larger size camper with a large payload, but otherwise intends to stay in places like Europe or the US and drive mostly asphalt roads, there might be more suitable vehicle choices available. Not that these trucks won't do it, so go for it if you've fallen for their charm as I have, but at the same time it would somewhat negate their advantages and there could be something to say about ride comfort compared to newer/other types of vehicles, oh and add consumption in there too.
Would not want a smaller tire as many campgrounds are muddy fields and I like not getting stuck.
