dal

I have a large filling that takes up about 75% of my tooth and the filling has cracked. My dentist feels that the tooth needs support and that a cap/crown is necessary. He also feels that although not necessary now, that I will need a root canal in the future given how big the filling is and how far down it goes. He recommends I either cap it now and may need to root canal it in the future (drill through the cap - doesn't sound good) or to do the root canal now. He feels a post would help support/anchor the cap/crown better since there is so little tooth left but said in order to put that post in, he'd need to do the root canal. Is this true? Is a root canal necessary in order to put a post in to support the crown? I DO feel some aching below the tooth at times (can't be sure if from TMJ or the tooth itself) so I feel I need to do the root canal.

More complications. In 2 1/2 wks I go away for a week. I'm trying to figure out whether to wait til after coming home (I chance that the filling he just replaced in the tooth will keep things calm and that the tooth trauma does not make things worse) - this filling is not fully formed so part of my gum seems exposed bet the tooth and the one behind it - or I go ahead with the root canal and post that can be done before the trip (but risk complication, pain and infection from the root canal - a previous root canal caused me strong pains on/off and sensitivity for months) and do the crown once back. I'm trying to figure out the least problematic of the evils. Not a clear cut decision!