OP says wheelchairs are not always available and suggests that is why he prefers to bring his own. U.S. airlines are required to provide them along with their code share partners.
When it comes to tipping, I said customary. My impression is that their salaries are barely enough to make a go. It depends who on they work for. My experience is limited to only one trip using this service at four different airports, BKK, NRT, HNL and ITO.