Part of the benefit of being retired is taking your time. You may not have a lot of money, but you have plenty of time. As others have stated, 250-300 miles a day is the max. This works out to 5-6 hours of driving. Stop 2 days at a campground instead of overnight. Find something interesting in the area you've stopped in and have a look.
See if there's a local restaurant that Guy Fieri visited on Triple-D. Call it the Eat Your Way Across America tour. Keep a log of local cardiologists.