Trouble finding Thévenin equivalent for this circuit
I changed the voltages from 20V and 5V to 25V and 0V
My resulting output voltage is 10V

Yes it is correct, but with respect to \$-5V\$
You forgot to offset the voltage by \$5V\$.
\$ -5V+5V = 0V\$
\$ 20V+5V = 25V\$
And when calculating the result substract what you added:
\$ 10V-5V = 5V\$ with respect to ground (\$0 V\$).