That article has a number of problems, some implied, some factual.
the basic idea is sound
but the need for such a test is questionable
and it could be harmful to the battery if appropriate care was not taken
Your RV battery bank has about 12 usable watt hours per pound of battery. The 'official' end of life point for the battery is at 80% or a bit under 10 usable watt hours per pound. Most RVers don't notice the battery being deficient until it gets well below that.
If you have a good converter, one that does multiple stage charging and has a maintenance mode that implements a sulfation inhibiting technique, then your outings will at least start out with the batteries with a full and complete charge. With experience, you will be able to gain a very good idea of the battery condition by how it fades on your outings.
Check the SmartGauge Electronics - battery metering
tech notes and also look at the description about how that device works. It is a much more refined means to obtain battery condition using the same idea as the above link.
Also keep in mind that any battery capacity measure with a precision of greater than 10% to 20% is exceeding the accuracy of the phenomena involved.
As Brian notes, the 20 hour rating for battery energy used a load of about 1 watt per pound of battery. Larger loads than this will yield less capacity, smaller loads more. See the Smartgauge tech pages on Peukert to learn more about this phenomena.