I have a winter home 10 miles from McAllen. This area has many RV parks for winter residents. They will be almost empty this time of year so you will have your pick of sites and parks. The area is very hot in the summer so I suggest you try and locate a site in a park that has shade for the RV. Tip O Texas is a good park but there are others. If you have only one AC unit your choice of a shaded site will be critical as your single AC will not be enough.
Creekin gave you a good route.... I have used it many times. Just be aware that there are no services for 100 miles between Fort Stockton and Sanderson on US 285. Also Sanderson is small and between Sanderson and Del Rio there is not much in the way of services.