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Determining how long you can boondock in your RV depends on 3 Key Factors:
The first two factors that we believe limit your stay are freshwater and wastewater. The third factor is your battery capacity. The reason we believe the tank capacity is the first thing to determine is your batteries can be recharged via solar or generator if you have no hook ups. Sometimes you may be at a campground with electric only, electric & water only and no sewer connection. Water & Electric only type of site is very common on river or lake front sites and older state parks. When traveling and stopping overnight is another great reason you need to know this information. We love to stop at Harvest Host sites and many have no hook ups.
The first thing you need to determine is your freshwater supply. We used this method to estimate how much fresh water we use during a typical day. This includes drinking, cooking, cleaning, and showering. We bought a water flow meter and measured the total gallons we used over a period of 5 days while connected to full hook ups. Then we determined our daily usage by dividing the total gallons by the total days 5. This gave us our average daily usage. We used this flow meter from Amazon.
We use on average 12 gallons per day.
The second thing we had to consider is our wastewater capacity. To determine how long we could go without needing to dump our gray and black tanks. We did this at the same time we determined how much fresh water we used on full hook ups. I dumped all our tanks on day one, then waited until our bathroom gray tank had to be dumped. That is the tank that fills the quickest since it connected to our shower. We now know we can go for 4-5 days on our grays tanks and 9 days on our black tank.
The third key factor is your energy consumption / battery capacity. for appliances like lights, fridge, heating/cooling, and electronic devices. Determine how much power your batteries can provide before they are drained. Lead acid batteries should only be depleted to 50% before damaging them, Lithium batteries can be run down to 20% based on manufacturers recommendations.
You can test your battery capacity while doing your tank tests. Remember the batteries only provide 12VDC power to the lights, furnace, fans, thermostats, and appliance control boards such as the water heater, and RV refrigerator controllers. You may have an inverter which uses the battery 12VDC and inverts is to 120VAC to run a residential refrigerator, and few receptacles in the RV. Most RVs the inverter does not run the microwave or AC units. There are some so confirm this before you need these appliances.
We use this Victron Smartshunt to measure our battery capacity. I really like Victron App with Bluetooth connection to my smart phone. This tells me exactly what % my battery capacity is. I also installed a 300 am hour lithium battery to extend our capacity.
We also have a generator to recharge our batteries if we boondock for multiple days.
We like this generator because it is dual fuel and we can run our AC unit with it. 
There are some other things to consider:
- Food Storage: Consider your food storage capacity and plan meals accordingly. Non-perishable foods are great for extended boondocking trips.
- Propane Supply: If your RV uses propane for cooking, heating, or running appliances, calculate how much propane you have and your typical usage.
- Terrain and Climate: Boondocking in hot weather may require more energy for cooling, while cold weather may require more for heating. Also, consider if you’ll be traveling through areas with limited resources like water sources.
- Conservation Practices: Practice conservation techniques to extend your boondocking time, such as taking shorter showers, limiting the lights used and devices being charged, and minimizing waste. Remember you have to take your trash with you and dispose of it properly. Please leave any sites you use cleaner than you found them.
By considering these factors and planning accordingly, you can determine how long you can comfortably boondock in your RV. Keep in mind that experience will also play a role in refining your estimates over time.
I hope you found this helpful, and I wish you safe travels and happy RVing