Try Fishtale River Guides (Wasilla, Little Su River) for salmon in August...might be before the peak of the run
Regal Air offers day trip fly-outs from Los Anchorage
In reality, the options are too numerous to mention: There are quite a few land-based options in Talkeetna, Wasilla, Soldotna, Cooper Landing, and Kenai...all on the road system and (more or less) accessible with RV's. There are various roadside options along the Glenn from Glennallen to Palmer and there are day-trip lake fly out options also along the Glenn (suggest Copper Valley Air...I believe they base a float plane on Tolsona Lake in the summer).
For salmon, you'll probably have better luck (based on your overall trip time) later on your trip and more towards Anchorage / Kenai Peninsula than in the Interior (Fbks, Tok, Glennallen). Trout that sit in lakes tend to get a bit lethargic during summer, but (depending what the regs bring for this year) should have access to trout, char, dolly fishing in the lakes and streams between Glennallen and Anchorage. If you take the Parks, the roadside (i.e. just stop and drop a line) fishing is not as prevalent, but there are more (Montana Creek, Willow, etc) locations on the Parks where you might have some roadside salmon action...but best bet for salmon is to get a charter (air, boat, or saltwater).
CRL
My RV is a 1946 PA-12
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