Regarding availability of plans and diagrams, to see what is possible, go over to the Winnebago site. Seems like they have detailed engineering drawings for every system in their motorhomes, as pdf's, available to the consumer.
Now, Winnebago is not perfect. One can find posts about all kinds of problems with their products, but at least the information is available.
Winnebago's profit margin may be such that it allows them to record and post such information, but I think it has more to do with corporate culture and what the organization thinks is the right thing to do.
Does it cost more to do that than to not do it? Probably. Is it a lot? Probably not.
Why doesen't Jayco or Starcraft or Forest River or most everyone do the same thing? They don't want to. Why? No idea. (could be bad advice from Legal, good advice form legal, a risk-averse culture, a bad experience x years ago, any number of things) They may not want to break that rice bowl for the dealer (read The Sand Pebbles or watch the movie with Steve McQueen...good stuff)
Not having drawings available makes me wonder if anybody really knows how the wires, plumbing, systems, Etc are actually installed. I expect someone, somewhere in the company does know, but they for whatever reason, are not telling anyone else.
2013 Starcraft 266RKS
1999 Ford E350 V10 Chateau (6.8L, 4.11 LS)
2005 Cedar Creek 31LBHS
2001 Sprinter 303BHS
1997 Mallard 33Z
1990 Starcraft Starmaster