I don't why one route should be much safer than another, even in a big RV. Traffic and speed are more likely to cause accidents than snow, especially if you are thinking of serious ones. A Thule fog in the California Central valley can be especially dangerous. In Washington, icy roads are the biggest weather related cause of accidents, and those are more likely on bare roads close to the freezing point than on snowy ones. I84 east of Portland through the Columbia Gorge has a worse ice reputation than the higher I90 over the Cascades.
Bridges ice up first. If I suspect the roads are icy, I pay attention to any wheel spin when accelerating, and test the brakes (when traffic allows). If your RV has ABS, read the manufacturers directions on handling icy roads. You may also need to carry chains.
Any of the freeways (even US395 between I84 and I90 is freeway quality) will be similar in ease of travel. Check weather reports and road condition reports. All the state DOTs have reporting systems online, and also have 511 phone reports.
The fastest routes that Google maps gives use non-freeway segments. All suggest US395 to I90 in Washington. One uses US97 in Oregon, another US95 in Nevada and Oregon, another US93 in Nevada. All can be cold, but won't be excessively snowy. I5 to Portland, and I15 to Missoula are nearly as fast - all around 19 hrs of driving time (by their estimated speeds).
