Here are some offical descriptions for the lights we are talking about which the regs seem to bundle together in
regulations ....
front and rear position lamps, (so these are the regular 'sidelights' that people are familiar with)
rear registration plate lamps, (number plate lights)
side marker lamps, (these would be the markers alongside a long vehicle)
end-outline marker lamps (these would be the lights in the top corners of a tall vehicle)
Are Side Markers needed on a Motorhome? I think it is one of these things where IF they are fitted, they need to be working, but if they are not present, it doesn't matter (so if say you had them and they were not working, it would fail an MOT, but if you removed them or put duct tape over them, it would pass).
Personally, I think they are a good idea and don't know why on a long Motorhome, it isn't needed but an equivalently sized 'working' van must have them. I guess some makers may add them for their own reasons, but not legally needed.
Example of a fairly long Motorhome without side marker lamps, but with end-outline marker lamps due to height ....
View attachment 64507
If this were a plain van of that length, it would have them for sure.