One option for dumping that most people overlook, Bus yards! The Mexicans have premium bus's that have all the bells and whistles, including bathrooms. So they have to dump somewhere. It might be just an open pit with a grating over it or maybe even a connection to the local sewage plant (I've seen both); but the yards where they are serviced and washed will always have something. If you ask real nice (like with a 50 peso note) you can sometimes arrange a dump.
Don't assume that just because your rig is older, that they will have diesel parts for them. We spent Xmas & New Year parked on the front lawn of a hotel in Tampico waiting for a fuel/oil pump for a 1998 37' Safari with a Cummins Diesel in it. The pump had to be shipped from the US. (Not my rig but our traveling companions, so the hotel had 2 big rigs parked on their front lawn for 2 weeks with extension cords plugged into the room we were using for showers, etc. Like someone else said, very friendly people down south)