When I took my Model S to Alaska last summer and had a leg that might use all my available energy, I did experiments ahead of time by driving at different speeds and noting the energy usage. While 65 mph was about 275wh/mi, I could get down to 240wh/mi. by driving slowly (like 40mph!). Thus, I had about 35wh/mi. improvement under my belt if I needed it. Why don't you note the wh/mi. you get at various speeds pulling the Airstream near Bakersfield and see what kind of improvement you get by at regular speed and driving slowly, just in case? Of course driving slowly will only help with aerodynamics and not with the physics of lifting the mass of your Model X and the trailer up thousands of feet. For that you need to do a test on a hill that is something similar to the grade you'll be ascending on the way to Yosemite. You might try various speeds and note the wh/mi. energy usage. Once you have these numbers, you will be able to plan a conservative strategy to get to Yosemite with the trailer.
There are parts of Interstate 80 over Donner Pass in California where my Model S, which normally is cruising at less than 300wh/mi was using greater than 600wh/mi climbing the steep grade. If I added the extra drag and weight of a trailer my energy usage would have been considerably higher than that, so you might find that doubling your normal energy usage while climbing a steep grade and pulling a trailer might consume considerably more than double your normal energy usage.
Another resource is the Tesla navigation system. Set your destination into the navigation system and you will see a turn by turn navigation guide to reaching that destination. Now push the "trip" button and you will see at the bottom of the trip window the reserve that Tesla's navigation system thinks you're going to have when you arrive at that destination. It should take altitude gain into account. At first the estimate might be quite inaccurate for towing a trailer up a hill, but as you progress the estimate should become more accurate. Now as you drive, keep an eye on the reserve. If you're heading from 30% to 25% to 20%, then things are clearly not going well, but if it stabilizes at a number above 15%, then you're likely going to arrive with a suitable reserve. The point is to watch the trend of the reserve number. If it is going up or steady at a good number, great. If it's going down towards uncomfortably low numbers, then you have to do some decision-making.
It is, of course, always a good idea to have a Plan B. If there are no regular charging stations along the way, see if there's an RV park with 50 amp charging along the way, and Plan B would be to pull in there for a quick "helper charge" if you're not happy with the energy in your battery as you approach it. The combination of having a Plan B charging destination and knowing how much benefit you get from various speed options should allow you to avoid ever running out of electricity if you pay attention.
By the way, do you have one of the new 30 amp adapters that Tesla has just started selling? Some RV Parks don't have 50 amp service and you will need the 30 amp adapter if you want to put in electricity at anything resembling a reasonable rate (it took me 3 days to charge a near empty battery in Alaska on a 110 volt plug).
Sounds like a fun trip, and you can do your fellow Model X owners a favor by noting your average speed, your average wh/mile energy consumption, and the number of miles of range you used driving from the Fresno SC to Yosemite with the Airstream. The more people report such things, the sooner we'll all be able to plan for such trips more easily. You should have the mileage available to arrive comfortably, but someone with a 60kwh battery might have a more challenging time of it and would really appreciate your posting your energy usage.
