Per Google maps I-10 and I-20 are both ~1,550 miles from Deming to Tallahassee -->
https://www.google.com/maps/dir/Demi...9!2d30.4382559
Based on my recent experiences, I'd think I-20 would be a bit faster route as:
1) Driving across Dallas-Fort Worth is a bit quicker than driving across Houston (I-10 gets very congested at the I-45 intersect).
2) I -10 between Beaumont, TX to the Louisiana state line is very slow with significant congestion. I-10 in this stretch is very rough.
3) I-10 is also horribly rough between Lafayette and Baton Rouge.
4) Traffic comes to a near stand-still on I-10 starting 3-4 miles west of Baton Rouge as I-10 / I-110 traffic merges to SINGLE lane!
5) There is significant construction on I-12 on the east side of Baton Rouge.
6) If you have concern about your motorhome climbing "hills", there are tall, rather steep bridges along I-10 as it crosses major rivers between Beaumont, TX and the Louisiana / Mississippi state line.
Perhaps others can share current experiences on I-20 congestion, construction, etc.
Hope this helps!