The idea that you should get a 30% discount from MRSP comes from guessing what is the dealer's margin on the unit, and how little of that margin might actually be needed to make sales and stay in business.
Manufacturers selling direct, the whole point is to try to keep prices lower by not having a margin for the dealer. RVs sold direct are still being sold at something over manufacturing cost, and sometimes are sold at a "factory discount" price lower that what is shown on a MSRP sticker or in the sales catalog. But that "discount" is not always negotiable. When I visited Casita off-season in early 2012 I was offered discounted prices on sales from stock.
This is not specific to molded fiberglass trailers. Several brands of motorhomes and premium trailers are sold factory direct, some from factory stock, some as built to order, and the sales people sometimes play the factory discount games.
On the other hand, some brands of molded fiberglass trailers (Bigfoot comes first to mind, but there are more) are sold through dealers, and you can negotiate with the dealer for a discount out of his margin. While Scamp and Casita sell factory direct, it is not the universal way of selling this type of trailer.
Tom Test
Itasca Spirit 29B