Road trip Portland OR to Las Vegas

Not a lot of time for the distance. I would haul straight down 5/99/58/15 and locate some RV parks accordingly. Rest stop or truck stop if needed. Best of luck.

Last year, on our Alaska trip we took 2 different routes in this directions.
From central Oregon down to Vegas you'll be driving via mostly deserts, so not much differences. Scenery was nice, but after couple of days become boring.
Icing on the cake was running into Hot Springs camp in So Oregon than hitting Death Valley with 124F.
We took advantage of cheap RV parks at casinos, but who knows if they will open by then.
If you like ghost towns - Goldfield is 1 of the best.
Than Rhyolite
"Shady Lady " is no longer in business.
Driving via Idaho will give you more green landscape and rivers, but for additional 200 miles.

Just came back to WA on US-93, more to look at than US-95. Gas in Nevada can be a challenge which ever way you go if you push your luck. Just keep the gague on the top half and all should be good.

Ironic. Any way, I’d find out what campgrounds will let you camp, and then decide the route.