Van did not shift correctly; very slow but finally did shift gears. Advance Auto machine showed code P1715. What does this mean?

Diagnostic trouble code (DTC) P1715 will set when a mechanical failure of the SSB shift solenoid is detected. When the SSB solenoid is stuck off, the symptoms will be incorrect 3rd gear ratio or no 3rd gear. When the SSB solenoid is stuck on, the symptoms will be 1st gear ratio incorrect or no 1st gear. The diagnosis procedure directs to check for other DTCs; if there are no additional DTCs for the transmission fluid temperature (TFT) or other shift solenoids, then to replace with a new solenoid and/or valve body. You can read about transmission solenoid replacement here: https://www.yourmechanic.com/article/how-to-replace-your-vehicle-s-downshift-solenoid-by-jessica-howe

bad speed sensor

Okay you will need to program the shift points again this is normal when battery voltage is lost, you will need to shift yourself and try to get it to shift when you want it to, what you do is remove the negitive battery terminal for 10 mins, then connect it back, start the van when you take off the shifter must be in low gear try to go slow until you have a straight away then accel and shift gears up until you hit the last drive gear make sure to shift as it use to or as close, dont rev to high before shifting as you shift up the computer stores the info and thats the way it will shift if you make a mistake unhook batttery again and try it again. i cant rememeber the exact shift points but i think you will know as its the van you drive, doesnt have to be perfect, for example start off in low and when you hit 15mph shift up a gear then ithink at 25 shift up and then 35 or 40mph shift and so on, you dont want the engine to rev to high between shifts it should be smooth but you you feel it a bit, hope this is helpful. let me know the out come, i do this all the time, dodge ,chrysler cars and vans. good day.

p1715 = Manufacturer control transission

Is it due for a trans oil and filter change?

well i had the same kind of problem with my 1994 f150 it is the part called the MLPS (manual lever position sensor) it slides over the gear selector lever on the side of the transmission, when replacing it you need to be sure to use the installation tool or it will have almost the same problem, the switch goes bad over time from water and dust because it is not a sealed electronic device

there inside the tranny you have to remove the pan to see them but thats not going to be your problem go buy a can of TRANSX first!! and try that if that helps then you need to plan on rebuilding your tranny or replacing it

good diagnosis that is correct if the vehicle speed sensor is not working the speedo doesnt work and the trans doesnt know when to shift

worn shift fork. shifts linkage could be binding.

Automatic transmission are complex systems. The shifting of gears is based on a number of things like bands, speed, acceleration, deceleration, manifold back pressure, RPMs, temperature, etc.
Sometimes the electrical ground connection for the transmission can become corroded which causes poor shifting.
Since you are getting an error code, you should get a readout for it. Many auto supply store like Advance Auto will loan a meter for you to get a readout. Call up some stores to find one.