How to Keep Food Crispy for a Potluck?

I wouldn't do anything crispy for a picnic. Sorry. You might want to rethink what you're making.

Putting it in a sterno tray will work. Putting the fried food in a crockpot will just make the food absorb the oil and be close to inedible.

Most crispy food won't stay crispy in a crock pot. It will stand a better chance of staying crispy IF you don't put the lid on the crock pot. The lid captures the moisture coming off the food and moistens the crispy coating.

Try to do a non crispy food.

Good luck to you and yours.

If there is any moisture or condensation in the container you store it in... it will get soggy and not stay, crisp.
So, before you put the food in whatever container you will store it in, make sure the food is completely cooled down, first. THEN put it in the container.
And have vent slits in it.

Don't add grease or oil, that will just make it soggy and TOO greasy.
Fried foods are best served fresh, I'd make something else. Though cold fried chicken is not bad, we had that growing up and it was always a hit. I think it works cold because it's not steaming and getting soggy in a warm container.

We have an awesome chicken place that we use for picnics, etc.
They put the freshly hot fried chicken in a foil lined white styrofoam cooler.
"Crunch" crispy? Not really.
But it does a pretty good job.

When you cover the food, use something that you can cut slits into, such as foil, to let the steam escape. Depending on when you cook and when it will be served, you'll lose heat and temperature. I hate getting any carry out with fries b/c they get soggy in no time, but places that do it right will either cut a hole in the top of the styrofoam container or not seal the paper bag. You will lose some crispness no matter what. Also try to avoid stacking too much food; again, anything that traps heat and doesn't let steam out will lead to sogginess.