Mountain Passes

Only questionable areas for some drivers, is dropping down into the Columbia River Gorge south of Goldendale.

Nothing is scary about Toppenish to the Columbia. If you have issue with heights the bridge over the Columbia will give more problems. The highway never has to loop back on itself to gain height. There are stretches with guard rails, but the drop offs are typical for highway embankments.

Speaking of 'dropoffs', the Saleh Creek Bridge on I82 (just north of Yakima) is regarded as one of more beautiful bridges in the state. You don't see much of this beauty from the bridge itself, but there are rest stops at both ends. You also get a distant glimpse of the bridge from WA821. (and for a brief while 821 is 300' above the river).

https://maps.google.com/?ll=46.700377,-120.444638&spn=0.015805,0.024033&t=p&z=15&layer=c&cbll=46.700377,-120.444638&cbp=12,0,,0,0&photoid=po-3825365

I was also going to suggest taking Canyon instead of I82. It dumps you back on I82 before you get to Yakima. If after that your ready to call it a day, check out the Yakama Nation RV park in Toppanish . It's a great overnight, (booze free outside of your trailer) with a casino in walking distance.
I have heard that the section between Toppanish and the Dalles is pretty scary so I haven't done it, but that was a while ago and I have issues with heights.

enblethen wrote:
...
And fire near Leavenworth
Leavenworth Fire

All businesses and visitor facilities are open in the Leavenworth area. While there may be some smoke in the area and an increase in fire traffic traveling through Leavenworth, Incident Managers want to assure visitors that Leavenworth and the surrounding area are open for business as usual.

In two weeks WSDOT plans on closing US2 west of Leavenworth (for a week to finish a bridge replacement), using Chumstick hwy as the detour. There's a chance that this fire could complicate matters. However it looks they are throwing a lot of manpower at it.

Fire burning near The Dalles as we speak.
Fire near the Dalles
And fire near Leavenworth
Leavenworth Fire

I blocked the section from Ellensburg to Yakima out of my mind. PaulJ is correct about I-82. WA sr 821 down the Yakima River Canyon is a great drive. It is easier on the rig then going over I-82. There is some nice campgrounds (federal) along the canyon route.
Sorry: Paulj

US 97 was closed for a while but is now open. If you're wondering about the highway, itself, no problem for a rig of any size.

WSDOT photos of forest fire along US97
Taken at the end of July

Passes should be no problem due to weather.
Way things are going you won't have to worry about the fires. they have already burnt off the area.
wsdot.wa.gov for traffic conditions.
WSDOT Pass info
Satus Pass is on US 97. It is not shared with I-82.
OP is planning on turning south on US 97 east of Leavenworth and going straight south to the Columbia.

In September weather is no problem. The first pass, Blewett, is highest, but generally regarded as easier than US2 Stevens Pass to the west. (Check WSDOT for possible closure on US2 in Sept if headed west from Leavenworth). The next pass is shared with I82, with several ups/downs. Traffic seems to travel as close to 80mph as it can. WA821 is the old US97 route through Yakima Canyon - flatter but winding along a scenic river.

US97 has another pass, Satus, in the south (after it leaves I82), with gradual climbs on both sides, and a state park near the summit. There was a forest fire on the north side of this pass recently, that closed the highway for a bit. But that is in the past.

WSDOT has a number of webcams along this highway.