Vacation food has a reputation of not being healthy, either because you're eating restaurant meals and fast food that are high in salt and fat or because you're limiting yourself to portable snacks that don't really give you a balanced diet. After a few days, all of these options can seem really tiresome. However, it is possible to eat a healthy diet while on vacation by looking at alternative cooking arrangements.
One option is to look for extended-stay motels. While these are meant for people who need to stay weeks and months, you can usually arrange to stay for a number of nights. These rooms have almost-full kitchens. Standard features include a full-size refrigerator, a cooktop, and a microwave along with a sink meant just for kitchen use (so you'd have a sink in the kitchen and a sink in the bathroom, instead of the typical motel setup of one sink for the entire space). Higher-end extended stays may have a regular stove/oven combination as well. You can go grocery shopping and have regular home-cooked meals that fit your dietary needs well.
Portable Small Appliances
If you're going to stay in a regular motel or hotel, where maybe you have a small refrigerator and possibly a microwave, you can try bringing in things like a portable grill. It's best not to use these in the room because spattering grease and intense cooking smells can make the room unpleasant for both you and future occupants. See if you can plug the grill into an outdoor outlet on hotel grounds, or try to bring a fully portable camping stove that has its own power source. Combined with the refrigerator's storage space, you can have relatively decent if simple meals.
Super Big Box Store and Supermarket Deli Sections
If cost isn't an issue for you, seek out the nearest "super" version of a big box department store or gourmet supermarket and head for the deli section. Many of these places have precut and prewashed vegetables and fruit, along with basic sandwiches and entrées. Many have premade salads where the dressing is on the side. With some careful choices (so you don't end up overdoing the fat and salt like you would with a sit-down restaurant meal), you can have an OK meal with this strategy.
Rent an RV
Another option is to forgo the hotels and rent an RV with a kitchen. Small RVs can be middling in daily rental and fuel costs, but they do contain enough kitchen space and appliances to let you make full home-cooked meals.
If you're concerned about your diet when you go on vacation, contact the hotels and motels you're considering and find out what food facilities they offer. Some have varying levels where a cheap room has a small fridge, but a larger one has a kitchenette. Keep these tips in mind when looking for places to stay.