Your 40 gal tank should have 32 gal usable.
My 6.5W has averaged about .75 gal per hour.
So the tank should last for 40-42 hours of running.
The Discovery Owners web site
has in the Hints & Tips how to tie the 2 tanks together. It does cost to join.
But it won't help the generator LP supply any. :(
The main advantage of connecting the two LP tanks is that your COACH has access to the 24 gal coach LP tank and the 40 gal generator LP tank (both are water capacities).
Note that this will not extend the available LP for your generator, mainly because the generator uses liquid LP and the coach uses vapor, and it is a very slow process for the propane to transfer to the generator LP tank. However, this modification is very beneficial as you can use your 40 gal generator LP tank to provide LP to your coach appliances.