Well, first of all Mexico. I live 40 miles from Juarez, and I won't go there. Yeah, I'm a fraidy cat, but living this close to cartel land all my life, I've heard the stories. My best friend from high schools big sister vanished down there in the 70's. Its not 1968 anymore in the frontera. Things did get mean. But not every tourist gets abducted, far from it. The real danger zone is the last 50 miles right at the border. That can be dealt with in various ways, like hire a ride to farther in.
But at the moment anyway, you can't go there. This quite well may change by winter, but at the moment, you won't be allowed to cross. And almost nobody is vaccinated in Mexico.
Charging. You will very likely be spending 6 hours of daylight time charging. Camp spots seldom have a plug.
But that leaves, even in winter, 6 hours of daylight left to ride. Maxing out range at 15-18 mph, you will have time to ride 70 to 80 miles. You ever spent 10 hours in the bike saddle per day? I believe you will welcome a stop after 6 hours. There is a good reason bike races seldom exceed 6 hours daily riding. Cant continue once your ass is a bloody open sore.
Out here in the western deserts, you will need 80 mile range. Plugs can really be 70 miles apart. You will welcome any kind of affordable campsite that has electricity. In New Mexico where I live, 14 bucks a night at a state park gets you an RV site with plug. Since covid, its by reservation only though. When I did some shorter camping rides, I used the state park camps when convenient. Otherwise I just stole power. The smaller towns tended to have city parks with plugs that were on all the time. I just looked for those. City parks, community centers, and also just free range plugs. I learned to look for real estate signs. Vacant commercial properties were the best bet. I never actually had to beg power at a gas station or such, but that has been done a lot by others that rode multi thousand mile tours. Occasional stop at a motel can be worth it, for the shower. Shower is another reason many of the NM state parks are worth the 14 bucks.
Lastly, there are ES members. You can definitely charge at my house, if you ride down hwy 70 in NM. Look up some others that may be on your route, and use them for a charge and a shower.
To do the charging, I cannot stress enough, GET TWO satiators. This gets you 700 watts of charging flow, into two batteries at the same time. Its a high flow, but not enough to blow the breaker on every plug you try. Those satiators can run while it rains, a big plus. And they are fairly small, can be bolted right to the bike frame, without water damaging them. Seriously, get at least one! Two typical 200-250 w chargers is TOO SLOW. Don't bother to get them 100% full unless you must, or its overnight. Charge to 90%. A satiator will do a 90% charge of a 1000w hour battery discharged to 20% in just about two hours. Just over 4 hours to get almost 100% full from a 10% charged starting point. So that can be a 4 hour charge, not a 6 hour. That's about right for lunch and a good nap. Or grab two hours here, two hours there when plugs are closer. Definitely stop and charge when you can. Ride into the night if you must.
Climate. In mid winter, like January, you really want to be in southern AZ, southern CA. NM is too high at 3500 feet. Its actually cold desert here in January. But by valentines day its fine in the south end of the state. The perfect time is october november.
The bike. Just a regular mountain bike is very difficult to carry it all on. 80 miles worth of battery will just about max out your weight limits on panniers. Yes, you do need 2000 watt hours, for the desert stretches. It really is 70 miles from plug to plug on some routes. I built a couple of longtails just for this. But a bob trailer can work ok too. Carry the bulk of your batteries on the bike, and the tent and such on the trailer. Not a big problem towing, if you are keeping speeds mostly down to sub 20 mph. But you do have to slow down more on the descents. Its the rocky mountains out here, even south in the desert.