I have a mother-in-law who is 90, a right side stroke victim in an electric wheelchair and takes about a dozen medications a day. We have her in assisted living. Even though she is being cared for, she still needs additional help with laundry and personal needs. My wife is always concerned when we go on a long trip, especially the 9 week trip we have coming up. My wife knows in an emergency, I'll fly her home.
Personally, taking a strong willed 94 year old as a third wheel on a long trip will most likely ruin the trip, or at least take the fun out of it.
Find someone who can check in on her once a week, be it a neighbor of yours or hers or a professional company, even if you have to pay them. I know you're struggling with paying for new tires, but all expenses associated with my mother-in-law are paid out of her accounts. If she has the money, just hire someone once a week to stop by. You might also find a local church group, Elks, or one of the meal services that feeds the elderly could drop a meal by once a week.
Leave some documentation around (like her front door) for who to call in case of emergency. If you're in a small town, the local PD may in stop by and do some checks for you.