The biggest negative about campground reservations in Alaska is that it puts you on a schedule. You will just fall in love with some places up there and want to stay some extra days. While we had lots of campgrounds reserved for our first trip in 2008, we had mostly “No reservations” (and that worked really well for us) on the second trip in 2012 trip. We have just a few reservations for our planned trip this summer (2015). Just be prepared to boondock and be slightly conservative with everything and all will be good.
This sounds like I’m going to rebut some of what I just said but we usually make reservations whenever we plan to stay for a few days anywhere. It just makes life easier. For our trip this summer (2015), we plan for reservations in Dawson City (4 days), Fairbanks (7 days), Talkeetna (3-4 days maybe), and Anchorage (7 days). These all require advance reservations so I will call them. Both Seward (7+ days) and Homer (7+ days) will be in a municipal campground so no reservations are allowed here. Regardless, you have access to water and dump. Unless you skip a bunch of stuff or are hurrying through, you will need this much time in these places to take in the sights – some commercial and some natural.
Make these reservations as late as possible so you are not on a fixed schedule. You will still butt heads with the caravans occasionally. Usually, you can hang around an extra day or two to get in a campground because the caravans leave after just a couple of days.
Make some calls to various campgrounds starting about mid May (when many of them open or at least start early staffing) to try to get a feel for how far in advance you need to make real reservations. But don’t lock in your entire trip. Be loose. Alaska is the perfect place to accommodate flexible travelers. You are an RVer and therefore, about as flexible as anyone. Flaunt your flexibility!