Re: raised beds in proximity to old septic tank(s) leach fields. I have a couple of thoughts and a couple of ?'s.
Is this system still being used? If so, do you ever put any harsh chemicals into the system? I'd not be concerned about bleach used in normal cleaning concentrations, and most cleaning products. But, it's something to keep in mind.
If a septic system is working properly, and if the tanks are cleaned before any sludge gets deep enough that it can block the exit from tank to leach field, it shouldn't be a problem. If the system is sketchy, then all bets are off. I had a neighbor with such a system. When ever it rained hard, or she did a lot of laundry, the leachate would bubble up from the ground in her field, and her yard had a frequent sewage stench. Eventually, the town forced the owner to fix the system.
My garden backs right up to my leach field. The leach field sits on a berm that was created when system was put in to prevent ground water issues. I don't allow any vehicle traffic over my leach field. The system works well, and I pump the tank every few years. I am not concerned about contamination of my garden from the leach field.
I assume the cautionary advice that you would get from your town or your County Agricultural Extension office would tell you that your crops should be 75' from your system. (I think this is the code for well placement in proximity to septic, and septic to open water.)