How identify part numbers to get the right replacements?

I have a 2007 Keystone Cougar that needs several things (landing gear legs, wheel bearings, landing gear gear box) etc But I cant confirm the part numbers. Keystone's website doesnt seem to have any repair/parts info. E trailer lets you enter your year/model like an auto parts site but then only lists 2 or 3 items for that coach. (nothing that I need). There are no part numbers on my landing legs, the gear box says "Venture". but no other info.