I have driven across Texas many times over the years, however it has been a while since I did it in an RV (just getting back into things purchased my first clss A 6 weeks ago after 10+ years without an RV). Starting in southern NM here is the route I would take with what I think are the must see sights along the way and the most scenic route. This assumes you are taking a roughly parallel to I-10 sort of route and that you want to get off the interstate for at least part of the trip. Keep in almost everylittle Texas highway has a speed limit of 75+ (85 in parts of west Texas on I-10).
Get off I-10 at Las Cruces for must see stop number 1 if you have never been there White Sands National Monument just outside Alamagordo * be aware that the white sands missle range crosses Hwy 70 and sometimes the highway is closed then a missile test is in progress. While I consider White sands a must see, I can't see spending over an hour or so there so consider it a small side trip, but be aware I think they close access fairly early in the day (3 or 4 pm).
From Alamagordo go north and cross over the mountains,consider spending some time hiking and outdoors around Ruidoso then into Roswell. You must stop at the Alien Museum everyone does it only takes a few minutes. Stop in the city of Carlsbad for supplies and fuel, then go south to Carlsbad Caverns, it takes a few hours to do the big hike down into the caverns then ride the elevator back up,again a bucket list thing to do if you are in fair to good shape for hiking.
For optional bonus adventure points in spectacular scenery and no one else around when you leave Carlsbad Caverns continue south into Texas and just after you cross the state line take a left onto TX state highway 652 (was in good shape last time I was on it, but that has been a few years) which will take you to the modern ghost town or Orla (lots of abandoned mobile homes, etc.)
From Orla go south to Pecos and stop for the West of the Pecos museum, for bonus points if you have an extra couple of days take Hwy 17 south to Ft Davis in the Davis mountains (yes there are mountains in Texas), several good RV camping options depending on the season,Davis Mountain State park for one. Make sure to see McDonald Observatory and check their calendar for public star parties which are held 3 or 4 days per week most of the year. Also note the town of Marfa 25 miles south has became a booming artist colony in the last 10-15 years. If you do head down to Marfa stop in at the Chihuahuan Desert Nature Center and Botanical Gardens a couple miles south of Ft Davis, there are several hiking trails here to choose from ranging from 1/2 mile to full day hikes.
Now back to our regularly scheduled drive: or if you bypass the Davis mountains side trip the next point on the map is Fort Stockton. There is nothing to see here, move along unless you need a motel room in which case Fort Stockton has plenty, but not much else other than the usual truck stops and Super walmart. I have yet to find a place to eat in Fort Stockton that is any better than fast food.
Now is a good time to get back on I-10 for a few hours, (there is a great little roadhouse / steakhouse just north of I-10 on the west side of Ozona called the Hitchin Post Steakhouse, parking lot is gravel and should be big enough for any RV. Nothing worth a special trip, but good food, home made deserts, etc
If the weather is bad and you want a place to hold up and stretch your legs try the caverns of Sonora which are a few miles south of I-10 between Ozona and Sonora. The caverns are nothing compared to Carlsbad, but they are dry if it is raining and there is a basic RV campground right by the entrance, just no dump station or sewer hookups (they don't want to contaminate the caverns).
Now back to I-10 east until Kerrville, I always think of Kerrville as the first place you see green grass most years when driving east on I-10 as the cactus gives way to grasslands.
This is the next chance for a good side trip, consider driving north from Kerrville up to Fredericksberg (google it, there is a lot here the Nimitz museum, the war in the pacific museum (could take 2 days to rush through and not stop at every exhibit, lots of good german food, festivals, and that quaint style shopping (everything from the hot sauce store, to Jelly.com's headquarters to hand built folk musical instruments) you may not like, it is also one of the new wine growing regions with a hop on hop off wineary tour bus which departs from next to the war in the pacific museum) There are several RV park options in and immediately around Fredericksberg, if you need some political cultural stops there is also the LBJ Ranch oh and also way too many roadside produce stands,mostly selling peaches when in season.
Now back towards Florida, you can now either make your way back down to I-10 for the speed route though the major cities or continue to zigzag across east texas on Hwy 290, then up to TX 105 or US 190, lots of varied sceneary which ever route you take. One way or another you then end up in Louisiana, don't miss it and if possible stay out of New Orleans. Soak up some Cajun culture and some Cajun food (not that New Orleans creole stuff with too much tomato in it) which can be found south of I-10 across much of the state until you get to the Mississippi.
Stop number 1 for a taste of good yet affordable cajun food in Louisiana is an exception to the rule it is north of I-10 (by a couple of hundred yards or so). Pick either of 2 locations for Hollier's Cajun kitchen (the original location on Ruth St (old Hwy 27 route) has a large gravel parking lot most likely to have room for an RV, I have stopped there pulling a trailer before.) If you miss it don't worry you have another chance 2 more exits down at the same exit with Cracker Barrel, just go north and it will be on your right just after passing Lowes, again plenty of parking (even more than the original location which can fill up around meal times) as it is in front of an old K-mart which has been converted to self storage. They have a good buffet, but the real value is if you order off the menu, word of warning then they say large on their $9 bowl of seafood gumbo they mean it, don't order it unless you are hungry. Overall the food is nearly as good as many great name cajun places, but at 1/3 the cost. I am going to skip over much of the next 150 miles or else we would be here all day..... Cajun Food stop #93 Pat's on the Wharf in Henderson, this stop is 3 or 4 miles down a little side highway, and prices are 3-4 times higher than Hollier's (oh by the way that is pronounced Hol-Yays), but this was one of the places that put cajun food on the world map 50-60 years ago, so worth stopping for some good sit down cajun seafood much of which is caught on their own boat. From here on it is back to I-10 unless you want to take a detour across southern Mississippi and Alabama, if that is the case I suggest making your way up to Natchez MS on either side of the river, more rural on the west bank, more antebellum and cultural on the east side (oh and stop for donuts at the donut shop about 3/4 mile from the Mississippi river bridge) on the north side of the highway at the odd angled intersection (yes I have stopped here in a motorhome). Then cut your way across through all the interesting small towns and surprisingly hilly scenery roughly following Hwy 98 and 84 to Dothan AL, then down to Tallahassee,from there I don't know of a good way, so back to I-10....