TA Travel centers and similar truck-stop like facilities might be your best bet.
They cater to people spending lots of time on the road.
They often have showers and other facilities that go beyond regular retail restaurant/gas station
stops
travel.stackexchange.com/users/3678/cdkMoose
You can buy a microwave that operates on 12 volts.
They aren't as powerful as mains voltage ones (600 watts or so, compared to 1500 watts) but they do the trick.
You can buy them from places that cater to RVers or mail order.
Here is one example Note that even at 600 watts, the current draw is 55 amps.
You should plan on wiring it directly to your car battery (bypassing the car's fusebox) and also make sure your alternator is up to the task.
I.e., if you plug it into the cigarette lighter (which is usually 15-20 amps), you will blow the fuse.
I would also be negligent if I did not take the opportunity to introduce you to the wonders of under-hood foil cooking.
Perfect for tandoori chicken on the road.
travel.stackexchange.com/users/17153/RoboKaren
In addition to the other excellent answers, from personal experience when you are farther away from larger highways there are less likely to be large "truck stops" and travel centers.
However, gas stations are generally run by clerks who are usually bored and generally friendly, and they rarely if ever object to you using the gas station microwave for your own bring-in food.
Just as when stopping to use the bathroom, it's polite to buy some small item like a bottle of water, soda, souvenir, snack, etc, or just to top off your vehicles tank with gasoline.
That way you are officially a "paying customer", and it is extremely rare that anyone objects (or cares)
if you use their sink, microwave, bathroom, etc.
Indeed this can be a good way to save money and eat healthier while on the road, while still respecting individual business owners hospitality and good will.
When in doubt, just ask something like "do you mind if I use the microwave to heat up my lunch/snack while I'm here?
" If you do this while in line to pay for your gas/small items, I have never been turned down or given a disapproving look.
Just clean up after yourself and you should have little to no trouble, and if they say "no" then you can likely just drive a block or so and try again at another station.
travel.stackexchange.com/users/12977/BrianH
I bought the Microwave and we use it on long trips.
It was a little tricky getting the connection to the battery, but once that was done, it works very well.
It also can be used in motels when you stop.
I went back to find a link for you guys, but the AC/DC model is now marked as discontinued.
They still sell the DC only model.
Here is the link.
http://www.roadtrucker.com/12-volt-microwave-wavebox/12-volt-microwave.htm .
It's not cheap, at $300, so before spending that kind of money you might want to take a look at the other cooking items they have which are much cheaper.
Also they do take a lot of power, but with the high output alternators in the cars today you should not have any trouble.
There was a little stove that we bought about 5 years ago that was only about $30, maybe $40 bucks and that worked pretty good.
You could plug that into the lighter.
It took a little time to heat up food, but you could keep driving until the food was ready.
travel.stackexchange.com/users/31296/Mike B.
Whole foods almost always has public microwaves in the eating areas of the restaurant.
Next sentence I also have found 7-Eleven sometimes have customer accessible cents
next sentence
I do that I'd eat healthy on the road all the time
and I'm always looking for my Ruede since I bring my own food next sentence courteous and buy something that's expensive specially at whole foods next sentence another place it's
another sure when our place is college campuses we have public microwave for student so if you go to a college campus somewhere and find the cafeteria specially community college there's always a microwave there so to summarize since I do this all the time
and I'm all stars in my car for
lunch Whole Foods always has them and college campuses always have them
so I make a point of working at those places
also some employers have accessible cafeterias that might grocery store
travel.stackexchange.com/users/40451/Greg Borden
Some coffeeshops may also have them.
For example it seems that some Espresso Royale branches (Midwest) have microwaves.
For chains like Espresso Royale, I don't think the staff there will really give a damn if you microwave your curry for a few minutes; indeed, they probably won't even notice.
travel.stackexchange.com/users/8803/Kenny LJ
I found one at Arco gas station with a food mart.
I bought some drink and they let me use the microwave oven.
It's so much better than to buy food from some shitty restaurants: healthy, economy, good taste from own cooking.
travel.stackexchange.com/users/37461/Daniel
I don't know if this also applies in the US, but I've notices that Mcdonald usually have a microwave in their fast food restaurants.
I think they are meant for heating baby food.
travel.stackexchange.com/users/24850/Joa
I'll be driving almost non-stop (no hotels).  I have some frozen curry and other things I'd like to heat up as I travel on down the line.  Where is it possible to use find a microwave to use?
Where to find microwave on the road in the US
travel.stackexchange.com
<usa><food-and-drink><stopovers>
travel.stackexchange.com/questions/42740
travel.stackexchange.com/users/22996/easymoden00b
You can buy a microwave that operates on 12 Volts, which you can take with you on long trips. However, you may be able to use a microwave at travel centers  and similar truck-stop like facilities, the eating areas at Whole Foods, some coffee shops, or McDonalds. Some gas stattion attendants will be happy to let you heat your food in the microwave there. Many places have a microwave that the public can use, including gas stations and TA Travel centers. However, a microwave that runs on 12 vots is also available for use on long trips.
Many places, such as gas stations, TA Travel centers, coffeeshops and Whole foods, have microwaves for public use. Mcdonald also has them in some countries. There is a microwave available that runs on 12 volts and can be used for long trips.