The SD Zoo has lift-equipped vans that will pick you up and take you to another part of the park (down into or back up out of the canyon for ex.) They will call the van for any disabled guest in a wheelchair or not. (At the time we went, 2 yrs ago, I was still using a cane, no wheelchair yet.) Our driver was really nice, and basically gave us a little private bus tour along our route, telling us all the extra tidbits that don't come up on the regular tram-tour.
Also, they had a program where a disabled guest could bring a "carer" (their term for attendant) in with them for free. So you could get one person in at no cost for each disabled person in your party. We did bring medical documentation, but did not need it.
Sea World also had a similar program that was an attendant could get their ticket for 1/2 price. You had to go the the Guest services window next to the ticket windows to buy those tickets.