Hello hunting world.
I work at an RV park in the Texas Hill Country where we sell small bags for guests and regularly feed the local whitetail corn mixed with dog food.
This feed mix is not healthy for the deer and it shows. We even had to put down a baby buck who was very sick.

I've been doing a bit of research to find a cost effective feed with probiotics to present to my manager but haven't had the best of luck since maintaining a herd of deer is a little out of my scope of practice.
So any recommendations would be greatly appreciated.
Also a good seed mix to throw out in full sun would awesome too.

Thanks in advance.

Don't feed them. It's simply a bad idea.

Wild life need a variety of foods. If they start getting fed by humans they will stick with the easy meal and not get the variety they need. I am not sure about the south, but in the north people have kept corn feeders out and deer have been found "starved" with bellies full of corn. From what I read they do not digest/process the corn in the winter like they do in the summer so they shouldn't be getting it. They don't know any better and keep filling up on the easy meal.

No offense to the OP but this sounds like a bad idea on many levels. If you want to figure out what to feed deer, ask your state's Fish and Game/Wildlife biologists or wardens. Chances are they will tell you not to do it or that it's not legal, etc. If they do give you the go ahead, they should be able to recommend something that is healthy for deer. Instead of having tourists hand feeds the deer, you might consider a food plot. Another thought is what will happen when more predators come around, especially with sickly deer and start eating the deer? That probably won't go over well with your guests.

Feeding deer is illegal here.

Google "Creep feed", The problem with feeding is the gut bacteria change with the feed. The Deer having to reset their guts constantly is bad for them. The second reason it is bad, is that when they gather disease and parasite transmission is higher. Feed them off the ground, though by now it is probably too late. Feeding them close to home is just plain dumb, rodents show up and proliferate. The ground becomes saturated with parasites.
I only feed them when the temperature gets down to single digits for weeks, to give the younguns a little help.

Let's start with the corn. Corn is a carbohydrate source but does not contain enough protein or many of the other nutrients a deer needs to survive and thrive. Too much corn can upset the balance of the microbes in the rumen, and can lead to digestion problems, which in turn leads to larger health issues. It would be the same as trying to raise children feeding them nothing but candy corn. Not bad as a treat but should not constitute a significant part of the diet.

Dog food is meant for dogs, which are not ruminants. Completely different digestive system and different needs. There are hundreds of varieties of dog food, and problems could vary with each type. They could contain too much protein. Will certainly have some negative effect on the bacteria culture in the rumen. Likely are way off on the essential vitamins and minerals a deer needs.

Don't know what your area is like right now, but we are extremely dry in mine, so if the deer are finding feed, they are probably becoming very dependent on it very quickly until spring green-up. During drought, especially this time of year, seeing deer in poor health and deer dying is not something that is unusual. Hard to say if it might be from the feed or mix or not. If the deer are that dependent on the mix, then they would probably be in poor health/starving anyway. Just know the mix isn't helping them.

My suggestion is if you are going to feed deer, feed them what they need. Lots of places here in Texas sell feed specifically formulated for deer, and as far as feeding deer goes, that is the best option. If for some reason deer feed is not an option, goat feed will work. Anything formulated for sheep will not, as deer need copper and sheep are highly sensitive to copper in feed. Alfalfa hay can be something valuable to keep out, as it will keep the rumen functioning properly and meets many of the nutritional requirements of a deer.

If I were planting something at an RV park, I would look at plants like Englemann daisy, compassplant, Maximillan sunflower, bush sunflower, and maybe even Illinois bundleflower. These are native plants to the area, and they are beneficial to deer, pollinators, and birds. They would take a little bit to get established, and would require protection from any grazers, even rabbits, when they are young. These would also make great additions to a butterfly garden if you wanted to mix in some additional forbs that flower at different times. You would have to put them in areas that are not mowed until everything has cured for the winter, and even then, not nearly as closely as you would mow most turf grasses.