We made reservations from our RV park site in Wasilla 4 days before we arrived in Denali NP (we stopped in Talkeetna for a couple of days in between those two stops). For Jasper NP Whistler's CG, Lake Louise CG, and Banff NP Tunnel Mountain, we made on-line reservations 5 days before arriving in Jasper. Those were the only reservations we made in advance for a 3-month Alaska trip and I can't remember not getting in to our first choice stopover site any night during that entire trip. We used a combination of private RV parks, national, provincial, and state park CGs, Indian casino parking lots with hookups, roadside pull-outs, and Walmart parking lots during that period, but we never stayed more than 3 nights in one place.
Note that many towns in western Canada and the Pacific Northwest have RV dumps (some pay, some free) at city parks or fairgrounds. And in Alaska, many Fred Meyer stores/gas stations have free RV dumps and water, as do select gas stations (pay or free with fuel fill-up) in popular RV towns like Tok, Anchorage, Fairbanks, Wasilla, Palmer, etc. See dump locations
HERE.