how is rt 140 from Medford Ore to I-80 , is it hilly , gonna be driving older rv across country

It is spectacular, but I wouldn't drive any older vehicle
due to lack of services/people. I've driven back to KC from the RV several ways. Take I-5 to Ca 89 at Mt. Shasta and head to Susanville, then meet up with 395 and hit I 80 at Reno.
Head S. on I-5 to Ca. 20 at Williams and head East to intersect
with I-80 east of Sac and Roseville. Head all the way down to the I-5/I-80 junction.

so 89 to 395 takes me away from donners summit, correct?

yes. You come right into Reno.

I should add, have done 89 hauling a trailer
so it is entirely not too stressful.

so that helps a lot , thanks , now how bad is the Laramie to Cheyenne section ?

There is a great rest area at the summit between Laramie and Cheyenne. There is a bust of Abe Lincoln at the high point in the rest area.
US 395 into Reno is a lot less scary than Rt. 140. You still want to top off in Lakeview. Last time I went through Susanville from K-Falls (February) I used 138 which was also OK.

He wouldn't hit Lakeview if he went down to Mt. Shasta, Tom.

so how is the drive from Laramie to Cheyenne, is it that bad or what? is the grade gradual or straight up?

The grade on I-80 EAST of Laramie is NOT "straight up" by any means (I know because I was raised there...you should have seen that road before they built I-80)...it is a long grade with a few winds (rhymes with blinds) and at the summit (which is lower than when it was simply US 30 and the road went right past that bust of Lincoln)) you are at the highest point along the entire route.

Beyond that to Cheyenne it is a pretty level drive for the most part and then, of course, you are soon in Nebraska.