I’d agree with a lot here, but there is a LOT.
1 thing I didn’t see mentioned, ratchet strap(s)… As others have mentioned brake chamber winding off, sometimes is it not easier to strap the wheel up? If not going far that is, and you shouldn’t be with a brake down anyway.
But, basically, electrical seems to be the thing, bulbs, fuses and tools to replace both - if a Merc Actros then get spare headlight bolts, because they’re ■■■■ and break, you know the ‘twist 90 deg’ things to change the bulb.
They do an Emergency Susie repair kit, not sure how it works out price wise compared to carrying a set of leads and possibly not much use if you break more than 1, but might be a space saving option. Can probably get it cheaper than the link, I just googled that, also, make sure you get the correct 1, I know someone was showing me a kit for his truck and it was the twin line like the link, not the splitter version he required due to driving a Renault Premium.
If relying on sat nav, either wire up your own ciggy lighter or take replacement / repair, as some in some trucks are ■■■■■■ and also some have been abused by kettles and inverters etc.
cieranc:
cav551:
The most useful tool in any fitter’s tool box is a mobile phone - because there is someone on the other end of it who knows how to do the job.
Not a 3.30 AM on a ■■■■■■■ down Sunday morning there isn’t.
There is, it’s YOU 
Own Account Driver:
I keep a Wabco tyre seal set as well for punctures. It’s a quite expensive one shot deal but works and saves time and money if you need it. Tyre valve cores and the removing tool. I also keep the stab in tyre string type puncture repair kit which can get you out of trouble as well. I don’t carry tyre levers anymore as too ■■■■■■■■■■ but you can change a tyre without a beadbreaker or a Cheetah by using the trailer landing legs to break the bead and re-seat the tyre by putting a ratchet strap around the circumference and tightening whilst inflating.
Brilliant tip, love it.
When driving around, just think ‘what if’ and if you’ve not got it, and you deem it to be worth having, then grab it when you can, don’t have to have everything on the 1st day, just when you need it.