Wow...that is some ambitious trip! I don't recall many folks posting about going that far on that diagonal for that length of time. Should be an epic journey!
If it were me, it would be, get south as quickly as possible, with minimal sightseeing; that can be done on the way back when the weather is far more cooperative. I would also want to avoid the major population areas, but that's just me. I would not want to go through the Boston to Washington Megalopolis at all in February. So...my route:
I-95 south through New England to I-495 around Boston.
I-90 from the Boston area over to Albany NY and
I-88 through the Catskills to I-81
I-81 south to Knoxville TN, picking up I-75 south to Chattanooga TN.
I-59 down through Alabama to Birmingham then continuing on I-59/20 through Tuscaloosa and Meridian MS. There you can take
I-59 further south to New Orleans and pick up I-10/I-12 or
I-20 west through Mississippi, Louisiana and Texas.
I-20 or I-10 (depending on choices above) west through Texas and then I-10 to I-8 for the trip to San Diego.
Most of this, I've driven at some point or another, particularly the eastern stuff. I always hated driving through or around the beltways of the major eastern cities, hence the choice of I-81 south. I remember when that was being built, and it was a blessing when it was completed and made it convenient for folks like me who hated the big cities.
For me, getting as far south as possible to avoid weather complications would be paramount. And again, if it were me, I'd plan on mainly hotels/motels on the way south, as you're likely to find minimal places open for staying in your RV. Once you get down to Alabama, then you're likely to find more facilities open and available. Then heading west, particularly on I-10, you're much more likely to find places to stay.
Yes, you can have weather complications on this (or any route), going across the USA. Sometimes a matter of finding a place to stay for a day or so and wait for whatever storm you encounter, passes. Heading south will hopefully minimize the chances of inclement weather.
Now...about that return trip... ! ;) :W ;)