I have a 50lb propane tank. It took 24 nights of camping to go through it. During all that time, I had the water heater and fridge running the entire time, I used the stove regularly for meals. You will be surprised how little propane you actually use.
I have a 40 gallon fresh water tank. Not taking showers, but not being excessively conservative, I go through less than a 1/2 tank over a 4 day trip. You will probably want to use the campground facilities during the day as much as possible.
I have 28 gallon grey and black tanks. If I use a 1/2 tank of water, the 2 tanks are about 1/4 full (that's using the bathroom sink for teeth and washing hands - it goes into the black tank). If you don't use all your fresh water, you'll be fine with your dirty tanks.
Battery is your weakest link. For me, before I got the AGM battery, I used AA puck lights for stuff like reading, and the RV lighting (fluorescent) only for cooking, etc. I did leave the water pump on all the time. Could go 4 days max. With the AGM (group 31) battery, I can use the overheads a bit more (still use puck lights for nighttime reading), charge my cell phone, kindle and laptop as needed, and I still have almost a full charge.
For your first trip, be really conservative on the first day. Then look at your levels. If you're less than 1/5 down, then you're doing good. If you're more than 1/4, then you need to conserve more or find a way to increase.