1. Download a spreadsheet that calculates weight and balance. A bunch of RV builders have created them, so Google should find them.
2. Measure an RV, or have someone else do it, to get the distance from the wing leading edge to the items of interest (centre of mass of prop, engine, battery, etc).
3. Find the empty weight and balance for an RV that is configured similarly to one of your configurations of interest.
4. Modify the spreadsheet to add several lines where you can add or subtract weights at the arms you found in step 2. You can also add the ability to modify the arm of the battery, to simulate moving it to fore and aft positions.
5. Spend hours playing around, looking at the effect of adding and subtracting weights.