The Good Sam Trip planner with work. You can break the trip into shorter legs.
The shortest route is probably up US-77 to I-10 at Schulenburg, east to I-12 in Baton Rouge, north on I-59 at Slidell, La to I-75 at Chattanooga, TN to I-81 at Knoxville, TN, to I-84 near Scranton, PA to and up I-91 from Connecticut to a cut across highway in New Hampshire.
I would prefer to avoid that route because I don't like driving I-10 across east Texas or Louisiana, and would not want to follow 75/81 through TN and VA. But I haven't driven that route - someone else might be more familiar with it, and that could be a fairly flat route.
I've taken I-84 out of Connecticut to Dallas via Dayton OH, Louisville, KY, Nashville, TN, Memphis, TN, Little Rock, AR, Texarkana, AR/TX. Not bad hills anywhere along the route.
If you go north from Harlingen to the Dallas area, I'd recommend you follow US-77 north to US-79 - thus avoiding the traffic delays near Austin. (Though sometimes I use US-281 to the west of Austin).
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