Multiple wires going to the BAT connections and the extremely poor strand management there is a recipe for failure. Personally, I’d say take that back and have them correct it - I would consider that absolutely unacceptable work.
The non-vertical install and etc. isn’t all that uncommon, and is technically okay on a small unit like that as long as there’s no issue with moisture (since its current orientation negates the IP rating of the unit’s connection points) but the wiring there is certainly unacceptable, and unsafe.
Interesting that Motorhome dealers in the UK aren’t much better than here in the US. I have seen DIYers do better. As you have already been advised get those wires fixed. As far as the mounting orientation it should be vertical but with proper ventilation that could work. It would be better if it was at least at angle to allow heat to dissipate.