Idaho

The answer depends a lot on where in the state you want to be. The Sawtooths will be just starting to melt out, although 75 and 21 should be open with no problems. McCall is a little lower and should have some campgrounds open. Southern Idaho, along the Snake river will be fully open and accessible - just be mindful that the dirt desert roads might still be too wet. The campgrounds along Hell's Canyon will be glorious.

Travel by Motorhome in problematic weather should include this easy answer:
Maintain a day's supply of fuels.
Maintain a two day supply of water.
Have provisions onboard for three days.
Watch the weather carefully and do not push into a bad situation.

If the weather goes bad, hunker down where you are. It is called a "Motorhome" for a reason. Let them clear the roads and those that absolutely have to get somewhere go there. You can watch all this in the comfort of your chair while listing to the news/weather and drinking warm coffee (or other).

We used to do most of our traveling in the early spring and late fall. We often had to hold up for a day. Food can be harder to than fuels. Below freezing water can be an issue.

One year we were headed home after a family visit to the east and got stopped real late in PA. This was with two kids and in a car. We got the last motel room and the last pizza and counted ourselves very lucky. That was a real good lesson.

The fall of several years ago we might have repeated it in Iowa, but we got to a rest area as things were shutting down. We were ready. We sat in the warm coach for about a day and an half and were not displeased with out choice.

Matt

One of our daughters attended the University of Wyoming for undergrad and doctorate and it snowed every year for finals in May. We rented a Class C in Denver and drove up for her graduation. We got 1 1/2 feet of snow that May and couldn't leave for a few days.

Idaho isn't the same as Wyoming, but pretty close. You'll have gorgeous days and you'll get some weather. As long as you're flexible, it should be a fun trip.

Several years ago I looped through Idaho in late May/early June. It was a cool spring with a late melt out.

The popular state park at McCall had canceled a lot of Memorial day reservations. We came just after than and had no problems with a campsite. And the trilliums were out in full glory. McCall is a relatively high area.

BLM campgrounds along rivers were open. Stanley was still coming out its winter slumber.

We got snow flurries at Lost Trail pass (MT/ID border), and a bit later when we cross the Washington mountains east of the Cascades.

In sum, campground and highway access will depend a lot on altitude along with the weather.

You should have no problems. Yes, you may run into late spring storms, but should not last long.
It alsom depends onwhat part of Idaho. Northern panhandle could have almost any kind of weather.

Considering the facts that Montana is in the middle of a record cold and snowy month and Colorado is in the middle of a record Blizzard. I'd say the chances of an early thaw in the Rocky Mountain West is decreased. But weather being what it is April could have record warm temps. So who knows?

We are planning a run up hwy 95 to Bonner's Ferry ourselves this summer. Leaving at the end of May but with other stops on the way so it will be June before we get there. I'm not sure just how crowded Idaho gets in the summer anyway. I lived up there for 10 months and didn't notice herds of tourists.