If you’re planning on upgrading your water tanks in your motorhome so you’re self-sufficient for longer, you could go with bigger freshwater and greywater tanks. 50 litres per person gives around a week with sensible use (i.e. 4 short showers, 2 litres of drinking water a day and 10-12 litres for cooking and washing).
Given the sheer number of places you can fill up, having massive tanks isn’t as important as it would be in the Australian outback, but if you’re planning on driving to some of the more sparsely populated European regions, or you just want to camp in the wilderness as long as possible without seeing other humans, larger is better.
200 litres would be ideal for two people, but adding 200 litres to a small campervan adds 200kg and it will definitely blunt the acceleration and could make cornering and braking noticeably different if the fresh water tank and the grey water tank are both half full with the water sloshing around. However, 200 litres is really the minimum if you have a large motorhome and you’re carrying 4 people.
You’ll need to consider where you will fit both the fresh water tanks and the grey water tanks, and how you’ll empty the grey water tanks. As you put more and more volume, the plumbing connections get more complex. It’s straightforward to fit one tank, but you might want to call a plumber for a hand with a complicated installation. Plus, if you also have gas in your motorhome, you’ll need a gasfitter to certify it.
When adding heavy items to your caravanette or motorhome, the best solution is to keep them as low as possible. The same applies for water tanks, particularly because they will slosh around when half empty, and this creates kinetic energy that affects the braking and handling of the vehicle.
The tanks can go outside the vehicle, but if you are using your motorhome in winter, you risk the tanks and pipes freezing, and that can cause splits and leaks. If you put the tanks inside, you’ll have to find room for them, and that will compromise the storage (which is usually scant anyway).
Don’t put the tanks too far forwards otherwise you might overload the front axle. This positioning is also important if you tow a caravan because no more than 10% of the caravan’s weight should be on the tow ball.
When you go on holiday, check the manufacturer’s recommended tyre pressure. It’s usual that you would add another couple of PSI to the tyres if you are loading more weight into your motorhome as they will be compressed more with the weight.