RVing in areas with no campsites

Hi, in the early stages of investigating the purchase of an RV vehicle and I have some questions about the RV 'experience' rather than vehicle types.

I live in Mexico in an area with no RV resorts or facilities. 2 young kids. Think I would probably use it for weekend trips on the most part and occasional longer trips. Mostly to remote beach locations.

How realistic is it to do this? What do you do about water, electricity supply & sewage disposal when you are not staying in paid campsites/resorts?

My guesses to these questions are: -

1. Scrounge water at the nearest gasoline station? How long will a 40 gallon water tank supply last in a typical RV scenario? I know it wouldn't last a day in a typical day at my house but imagine water use might be less when travelling in an RV.

2. I see a lot of RV's have generators so I guess electric will be done with a gas/diesel generator. On average how many gallons of gas/diesel is needed for power in a 24 hour period with moderate AC usage?

3. I'm going to take a guess that a typical sewage storage tank would last a long weekend before getting full? Not sure what I would need to do for longer trips, can you safely dispose of your sewage in the wild without damaging the local environment?

I'm thinking a Class C RV would best suit my needs but have not been to a Dealership to look at the options yet. I would need to buy used and my budget would be around $15,000. It seems feasible base on my brief search on the classifieds.

1. Can you tell me what the average mileage I can expect from a typical Class C engine before having many problems?
2. Can you tell me whether most common problems can be fixed easily enough myself with typical Home Depot / Lowes type components or do you often need specialist expertise and components? Obviously this question is important to me since I live a long way from any kind of Dealer support.

Well, if you can take the time to answer these questions and offer any other advice you think might be relevant to my particular circumstances I'd be very grateful.

Have a great evening.