You can't get from heat lost in a week to the steady heat loss of a tank
being maintained at operating temperature. The heat lost every hour is what
needs to be replaced to maintain an even temperature.

Yes you can!

It's not linear so you need to do exponential calculations but YES you
can get it...

For the numbers the OP gave,

150 deg F initial temperature
cooling down to 80 deg F in 5 days
in 60 deg F air

I got 110 Watts steady state heat loss...

For that particular example, it takes 110 Watts to maintain the temp
at 150 deg F.

Mark