You are asking for the impossible. Costs are going to vary hugely depending on lifestyle. I am sure some couples living in a van conversion spend more than others in a Class A.
Here is where I would start. First consider "fixed" costs; e.g., RV insurance and payment if any, healthcare insurance, food, personal items, mail service, etc. We also had a $200/mo charge since we had a lot of items in storage.
Next look at direct travel related expenses: e.g, fuel, maintenance, tires, tolls. Again, this will vary all over the place depending on the number of miles you drive. We traveled a lot so this was a major item for us, around $1500/month.
The next big variable I would look at is the costs of camping. We had solar panels and rarely needed a generator. We almost never stayed in RV parks or expensive campgrounds with hook ups. Most of our stays were in National Parks with a senior pass or on BLM or equivalent campgrounds. We averaged $7/night. We also spent almost nothing on generator gas, maybe $5/month. We camped in warm locations so a $20 propane refill would last for a month or two.
We also expected to spend a significant amount on entertainment, entry fees, eating out, etc. That turned out to be insignificant with a senior pass giving free admission.