...Some good points made here, but one thing you have to keep in mind ANY time you ask for advice from anyone (especially on a public forum like this) on whether it is time to replace something expensive or not:
..Those whose bank accounts would not be affected by the replacement advice they give, will almost ALWAYS say 'yes, go ahead and replace now'. Regardless how old or worn the item in question is or is not.
However, watch those same individuals, on when THEY do a similar replacement. Amazingly, when its coming out of THEIR bank account, that replacement isn't needed quite as often. Its truly amazing how that works, haha. :)
I've seen this time and time again, on numerous subjects. Its just human nature, and something you have to keep in mind when you ask for this kind of advice. Its easy to advise someone else to shell out $$ to replace now, but when/if it is coming out of YOUR wallet, the perspective changes drastically.
That said, the general rule of thumb for "China bomb' trailer ST tires is around 4-5 years when its a good idea to replace them. I think you can go much longer, though, with a quality set of Maxxis (ST) tires if you take good care of them.
Having stored your TT indoors a lot is definitely a plus, as that means they shoundn't have been exposed to the sun as much.
I think if you're taking care of them and watching them as close as you say you are, you should be able to get one more year of service out of them. After that, I would replace them with a set of Maxxis ST tires which should easily last you 5 years or longer.