My last trip I mostly used 4 lenses.... 18-135mm, 10-20mm, 70-300mm, and 30mm f/1.4.
I got the most use out of the 18-135mm. Most of my shots were taken probably between 18mm and around 80mm.
For MK, EPCOT and DHS you can easily get away with using the 18-50mm (when ever my next trip will be I will be using an 18-50mm f/2.8 lens). There really isn't a whole lot at those parks that you need to zoom in close for.
AK you will get greater use out of your 70-300mm (I'm guessing you had a typo as I've never head of a 50-300mm lens). You'll want to to close pictures of the animals. I really only used my 18-135mm lens for some detail shots around AK, but mostly the 70-300mm stayed on the camera at that park.
At DHS you'll want to bring the 70-300mm to get some shots if you do any of the 3 shows,,,, Beauty & The Beast, Lights Motor, Action, and Indiana Jones.
If you want to get some upclose detail shots of the Castle at MK, then the 70-300mm would come in handy for a few shots.
I wouldl say that we here on the Photography board consistantly recommend a 3 lens package at Disney. 2 of those lenses would be your "kit" lens (this varies depending on your camera... 18-50mm, 16-85mm, 18-70, 24-105, 18-200, or something close to those focal lengths) a telephoto lens (70-300, 50-200, 70-200) and the last one would be a "fast" lens. This would be a prime lens that has a WIDE aperture... ie: 50mm f/1.4, 50mm f/1.8, 30mm f/1.4, 35mm f/1.8. These lenses offer you a great tool for use on the indoor dark rides (Pirates, IASM, Great Movie Ride, Haunted Mansion, Peter Pan, etc...) and for the night time parades (Spectro,
MVMCP Parade,
MNSSHP Parade). Because of the wide aperture on these kinds of lenses you can generally get a fast enough shutter speed (in conjunction with a HIGH ISO) to stop the action and get great images. Also you can't use a flash on the indoor rides and using a flash for the night time parade's will wash out all the great colors and make the images look dull and flat.