65 gallon water heater should fill more than 1 bathtub since typical bath only uses 12-14 gallons hot water. First hour delivery on your tank should be greater than 60 gallons.http://waterheatertimer.org/Hot-water-use-chart.html
Since you have such a small amount of hot water, it sounds like bottom element is burned out, or one of the thermostats is bad. There are two thermostats, and two 4500 Watt elements on 65 gallon tank.
If upper element was burned out, or water heater was not getting 240V electricity, then you would have NO hot water. So we know upper element is good and circuit is good.
Ordinary tank-type electric water heater is fully repairable by homeowner and parts are available at hardware or home center.
I suggest taking 30 minutes and doing troubleshoot before replacing any parts.
Buy cheap multimeter from home center.
When testing voltage, rotate multimeter dial to 240-250-277 VAC
When testing elements or continuity, rotate dial to Ohms.http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html
Add a comment for more free help. I can answer questions as you fix the water heater.
Also take advantage of fixya phone service.
For a price, fixya expert speaks with you over phone while you work on heater.