I think that if you have problems (health, mechanical, etc), Most of the caravans will leave you at that point. You are "on your own" and may be able to catch up with them when the problem is fixed.
We have made the trip to Alaska three times, always by ourselves, and have experienced no problems whatsoever. We met many other couples on the trip and have had many great adventures while on these trips. I don't think that the caravans would have allowed us to stay at one spot for as long as we wanted.
Remember, you aren't the only ones making the trip. There are many others that are also out there by themselves, looking for new friends.
Good Luck
Journey Indiana