Seem we forget that in the 60's, 70's and 80's when the majority of RV's only had one 12VDC battery we did just fine. It is the intense desire for all the fancy stuff that drives the larger requirements.
Example of how to estimate usage
Device Number Rating Daily Hrs Daily Total
in amps Used In Amp-Hrs
Lights 8 X 1.5 X 0.5 = 6
Stereo 1 2.0 4.0 8
Water Pump 1 4.0 0.25 1
Fridge 1 0.8 24.0 20 (rounded up)
Minimum Daily DC Total 35 Amp-hr
Furnace 1 7.0 3.0 21
Maximum Daily DC Total 56 Amp-hr
For maximum life, you should never deplete a battery bank lower than 50% so I would have at least 140AH battery bank (minimum safety margin). Closer to 200AH would be preferred (two group 27's or 29's in parallel)
That is how you determine your daily use and storage capacity needs.
Battery charging (entry level RV) - How much energy you can put back in daily will depend on converter type and time available to run generator. An older single stage converter will take many hours to recharge. A modern 3 stage converter unit will bring the bank back to 80% fairly quick but that last 20% will take a while.
So, as your needs grow you increase capacity and charging efficiency however the basic way to determine your needs is still computed the same way no matter how many modern conveniences you desire.
Useful table of AH requirements: Determining The Total Amount Of Amp Hours You Will Consume - Solar RV Panels