I use an organic ketchup that happens to be really low in sugar and is actually sweetened with agave syrup. But hubby is diabetic so we always look for stuff like that.. Mt. Olive sells pickles and relishes made with splenda as well, though I don't eat either of those, if someone does, maybe that'd be an option for them as well.
As for the mayo, I also don't eat tuna, but I've read here that some people just use their Fage (or other greek yogurt) to moisten their tuna with (who was the poster that cringed at the word moisten? lol... I think of that every time I use it
) lol