I asked myself the same question myself when we bought our camper trailer. Luckily back then I was given a fixed price list of all extras. At first I thought some of the prices were a rip off, but after investigating the cost of parts required for that ‘extra’ it turned out to be reasonable prices as labour costs ended up being very minimal. (Some jobs are just easier while the trailer is being built)
So based on this, I ended up ordering the majority of extra’s at time of order as it wasn’t worth my while doing it myself. Plus doing it this way, they included some of these ‘extra’ in for free when negotiating the deal.