New to towing Hwy 2 or i90 to Leavenworth WA?

Stevens is only 1000' higher. No biggie.

Stevens has more traffic between Everett and just east of Monroe. It is a two lane road in many area. It is in good condition. It is not designed for speed but it is a comfortable drive at or just below speed limit. Use the provided pull outs for speeding traffic.
I-90 and US 97 is not as enjoyable drive. You will have two passes on this route. Snoqualmie and then Blewet. Neither is that hard.
Another return route you may want to consider is WA 20 over the North Cascades. Beautiful drive!

Another option, assuming you are coming from the big Vancouver, is Hwy 3 (Crowsnest) over the Cascades, and US97 south to Wenatchee. The border cross should be faster.

On US2 the most significant climb is right before Stevens Pass. Before that it spends a long time following a river. And yes, traffic through the towns at the west end can be heavy, with commuters and vacationers (and skiers) (depending on the time and day). The east side of US2 has a more gradual drop, ending with a flat but twisty canyon run right before Leavenworth.

From the south, I90 and US97 is faster, but probably not worth the extra urban freeway when coming from the north.

US 2 is the shorter route to take. It's not too bad of a climb. Two things to note. One US-2 is going to be closed from Coles Corner to Leavenworth starting on Aug 25th for a week. Second, the North Cascades Highway is currently closed due to mudslides so traffic on US 2 may be heavier than normal depending on when they get SR 20 open again.

http://www.wsdot.wa.gov/News/2013/08/08_US2_TumwaterCanyonClosure.htm

http://www.wsdot.wa.gov/traffic/passes/northcascades

http://www.wsdot.wa.gov/News/2013/08/08_US2_TumwaterCanyonClosure.htm
Week-long US 2 closure west of Leavenworth starts Aug. 25

I would not recommend the Chumstick Highway detour for an RV. There's a tight curve and hill climb just east of the town of Plain.

https://maps.google.com/?ll=47.751098,-120.652966&spn=0.029489,0.071411&t=m&z=14&layer=c&cbll=47.751098,-120.652966&panoid=H_riChDs93srCSJqbhyHYg&cbp=12,315,,0,0