First of all, is your bath a 'plastic' type?
If so, then before sealing the bath should have been over half filled then sealed so that the silicone was put in when the bath had a mid-way point for weight.
Secondly, if you had big gaps, you should have use the above method, plus before the silicone, used foam filler.....which is not the best thing.
Also, if the wife is cleaning the tiles with things like vilkie etc, this rots the silicone , as does most cleaning products.
There is a problem as to the squareness of old and new rooms. I did a bathroom only yesterday where the bath sat neatly into the corner, but as the short side went to the wall, it simpily went away by half an inch.
So, fill with foam filler, cut foam back down , fill with silicone, remembering that it will be a big wide joint, do it slowly, lots of water/washing-up liquid, press it down to get a nice finish. Do again a day later to get the best effect.
You can fill with silicone first time instead of foam, then do again the next day.