You'll be fine. US-89 is a decent route for your RV. The toughest part is in the King's Hill area but it's not tough at all. The grades are not that steep and it's a very scenic drive through the Lewis and Clark Nat'l Forest. I will take my 42' Bus through there probably next summer without any worries. May is not a problem in that area as far as snow goes. That you'll only run into on the smaller roads up around Glacier. Going to the Sun definitelky won't open up until at least mid June.
Mark & Leann Quasius
2016 Cornerstone 45A
2007 Allegro Bus 42QRP (Sold)
2012 Jeep Wrangler Unlimited - Rubicon