I understand Hutches point, and such savings are more likely to occur in a mass productions environment, and there are some well known major manufacturers who go to great lengths to save fractions of a penny on components, but none of the UK caravan manufacturers can be classed as mass producers, they tend to batch produce in tens of a particular model, and will produce a wide range of models from the same factory.
A few years ago I saw quite a few manufacturers production facilities, and having seen their shop floors with the numbers of staples, nails and screws, and other components strewn across the floor I don't think they seriously look for these sub penny savings.
In terms of design process, caravan manufacturers will start a new model design with a very clear perception of the target customer group and the range of tow vehicles they are likely to have. This will largely dictate the MTPLM they need to aim for to make the model attractive to the widest possible range of customers.
In addition each manufacturer will have a clear idea of how much payload they need to allow for each model based on the number of berths in the caravan.
Armed with these (and other manufacture defined weights, loads and tolerances) they will design the caravan to bring it in at, or below the target MTPLM.
Part of the process involves choosing the size and load capacity of the chassis components. All the 90% of European caravan manufacturers use chassis parts from just two companies. These chassis manufacturers don't make bespoke chassis for UK caravan makers, instead they sell sets of standard parts. The caravan manufacturers will select the parts. The standard components chosen may end up having a slightly larger load capacity than the caravans designed MTPLM. This gives the caravan manufacture some scope to increase the MTPLM if the customer wants it.