You have it backwards, gettin'...
A pumpout hose coming from a fitting at the bottom of the tank will always be full of standing waste as high as the level in the tank...so it will be the most likely to permeate before any other. The line from the toilet to the tank goes to a fitting at the top of the tank, (at least it SHOULD BE!)...so while it's certainly vulnerable to permeating, it's not quick to do so.
However, it's never a good idea to replace only some of the hoses because, by the time you get the first one done, the next one stinks. So do them ALL and get it over with!
Unfrotunately your idea of cutting off the bottom 3' of pumpout hose is likely to leave you wth remaining hose that still stinks 'cuz odor spreads all over a section of hose. Since it appears you have access to the bottom half the hose, replace it with hard PVC and--if you truly can't get to the end that's on the deck pumpout fitting, wrap the remaining hose in a doulble layer of Saran Wrap...which must be ONLY the Saran brand if you want it to work. Do a search for Saran Wrap for more details about wrapping sanitation hoses in it.