Yuma /Winterhaven

the parking lot charges $5.00 for the day at the border. It is right at the border. No bus that I know of. It is a safe and secure lot, run by the Quechan tribe. From the lot it is about a block into Mexico. The wait to come back to the US could be over an hour or more at this time of year. At least most of it is shaded and has benches to sit on.
The casino is very nice, lots of folks dry camp there. It is on the same road to Mexico, about 2 miles from the border. Have never stayed in their lot, only in the hotel.Very nice with decent food,and great pools.
If you get into Mexico around 8-9am, spend 2 hours, then head out by 11am, you should be out before 1pm.Just depends on how many tourist are there that day. The longest I have been in line to get out is 2 1/2 hrs.and I hate! standing in lines..Another idea is go late in afternoon and head out about 5pm-6pm. Have done that, and it was a shorter line.
Dona

The parking lot at the border has now gone u to $6.00. You can't park there overnight. Quite a few folks that do what it sounds like you want to do, park the "Q" (Quechan Casino) parking lot. There is no bus or shuttle to the border. Also that six bucks is higher if parking an RV or camper van.

I agree about hitting Los Algodones early in the day and leaving early. Yes, you can walk all over Algodones. Some people park along the side of the road, too, rather than the parking lot. But a space is hard to find and I've always used the paid parking.