A 16 year old boat is long done with the heavy outgassing from plastics and such. "Shat smell" kinda confirms that is head odor and 16 years confirms it is time for new hoses. Might as well do the whole job and get it over with, use the premium hose, and check the holding tank VENT to make sure it is clear and in working order. With good ventilation to the tank, the odors from it stop as the bacteria mix in the tank gets oxygen.
And having a solar vent fan in the head can only help.
If a boat sits unused for some time, the salt water in the head intake line starts to give off real funky odors from dead sea critters. No real solution to that, unless you pour fresh water back in that line to flush it (and then close off the seacock) or you feed the head with fresh water, which doesn't have to be as profligate as it may sound.