You can certainly take your entire load in rear bags, but if you are going to tour much, especially if you are going to head for some real hills, I think you will be happier with front bags. Nearly three decades ago Bicycling magazine published an article put together by the folks at Blackburn where they had placed the load in various places and combinations along the front forks, handlebars and rear. Their conclusions led them to develop the low-rider front rack which has been standard-fare ever since. Basically, loads in rear bags or on rear racks tend to destabilize the bike and loads in front bags, especially when placed about the front axles, tend to stabilize the bike.

I usually place most of my load in my front bags (tools, H2O filter, stove, fuel, cookset, freeze-dried food, anything dense) and put the fluff in my rear bags. In fact, whenever I can fit everything into one set of bags, I just use front bags. As far as attaching the front rack, my 1982 Trek 720 doesn't have any mid-fork braze-ons for the rack either. Hardware stores sell plasticized u-bolts with plasticized plates that can be used instead. The original low-riders came with this hardware.

That said, the other commenters are absolutely right that 30 lbs in rear bags should not be a problem. If the cost of a front rack is a barrier, just use what you have with full confidence. An added bonus of a big (volume-wise) rear load is that you look bigger to the cars that are overtaking you so they may give you more room.