I've just spent the weekend decoding a NMEA 2000 GPS sensor. Garmin's manual specified the PGNs the unit transmits and accepts but no detail on any of the payload. I've managed to come across documents which have been helpful, but whats been most helpful is Maretron's product manual. They get into detail, at least with their GPS sensor, almost as heavily as most NMEA0183 devices I've had the pleasure to use. Only problem is, I need to figure out how to configure my Garmin sensor since differential corrections are disabled be default... I've been talking with a Garmin representative about the lack of technical documentation and am waiting for a follow-up.
When it comes to standards, the manufacture might be the entity who purchases the standard, but we, the customer, are actually paying for it.
http://www.nmea.org/Assets/july%202010%20nmea2000_v1-301_app_b_pgn_field_list.pdf
I've been active over at Seeed studio's forums. I've been actively developing and ironing out the MCP2515 library. I would recommend checking out my github if you end up using the MCP2515 with an Arduino.