Hello And welcome Kayden.
I'd guess that across all new caravans it's roughly 70:30 in favour of external submersible pumps. I suspect the reason is cost rather than any other particular clear advantage.
Some of the differences have already been mentioned, whether they represent a pro or a con is for you to decide.
Ray wood has suggested you would need to replace the external fitting of you change to an inboard pump. That's not necessarily correct. You wouldn't need the pump or its wiring part, but they could simply be removed, leaving hose to dip into the water carrier. You could replace the double hose for a single if you wanted to.
Because submersible pumps use a rotating impeller to move the water, if the pump between airlock (often occurs when refilling a water container) the pump may not pump water untill the air lock is sorted. It's easy enough by just unplugging the pump from the caravan as you drop the pump into the water, then reconnecting it to the caravan.
I remember the job involved to get rid of airlocks with the old footpumos, but the problem was gone when inboard pumps came on the market. The reason is the pump uses a diaphragm with displacement chambers which have ported valves and they could overcome airlocks by themselves.
Eric also correctly mentions pressure switched external pumps being damaged if the water container empties. This is true and occurs because most submersible pumps rely on being able to pump water for two reasons. The first is the motors need water passing through the pump to cool it. Without water, the motor will rise in temperature, and that will soften and distort the plastic of the pump housing. The distortion will compromise the seals around the motor and water will get into it, and that will eventually stop the motor.
The other reason water is needed by the pumps, is similar to the first but for slightly different reason. Yes the water will cool the motor shafts seal, but also the seal works better if it's moist. The consequence of a dry shaft seal is increased wear leading to water getting past the seal and entering the motor compartment.
Some other considerations;
On board diaphragm pumps use oscillating motions and consequently can be quite noisy, especially with the structure of a caravan acting as a sound box. By comparison, submersible pumps only use rotational motion, which is inherently quieter, and the are outside and not in direct contact withbody of the caravan. They can barely be heard running.
Switched taps or pressurised control.
Virtually all inboard pumps use a pressure switch to control them and there is good reason for it; By there method of operation the pump can develop quite high internal pressure pulses. If this was not controlled it would build up the pressure in the caravans water pipes and it would exceed the Pipeworks, and appliances safe working pressures. So a pressure switch is usally built in to monitor the pipeworks down stream pressure. You can use switched taps to control the power to the pump, but the pressure switch is still necessary.
Caravan submersible pumps cannot produce the same high pressures of the inboard pumps, so they are unlikely to compromise the caravans pipework. However many systems do still use a pressure switch to control the caravans water system. It's relatively easy to fit, and in my experience is less prone to go wrong compared to the alternative switches in each tap.
Pressure switches need water pressure to be able to turn the pump off. They don't like airlocks, or if the water container runs dry, they will keep the pump running. Diaphragm. Pumps will withstand extended periods of running dry, an being inside the caravan it's likely the will be noticed, and manually turned off, but the near silent external submersible pump may not be detected for a long time. The advantage of switched taps is the pump cannot turn on if the taps are turned off.
Another consideration is whether to leave power switched on to the pump over night or when away from the caravan. With a switched tap system as mentioned above, when the taps are off, the pump will be off, but with a pressure switched system the pump circuit remains ready as long as the water pump isolater switch is on. If the pressure switch detects a drop in pipe work pressure it will turn the pump on, and will remain on untill the pressure is restored, or the isolator switch is turned off.
Some caravanner's used to complain about being woken at night by the inboard pump running for a few seconds. The reason was the pressure switch had detected a drop, and it turned the pump on to depressurised the system. The cause was usually the owners having their water heater on all day, and turning it off at night. Just before going to bed, they would use a little hot water which would drop the pressure in the hot tank. The pump would run to replenish the pressure so both not and cold systems were at the same pressure. With the heater turned off it would cool down, and as it did so it's pressure would drop below the pressure switches threshold, causing it to turn on to replenish the tanks pressure. This would not happen with switched taps, and it's a good reason to switch off the pumps isolator at night or when leaving the caravan.
Another cause for sporadic pump activation with a pressure switch, is if there is a leak on the pipework. Check pipework runs for visible evidence of a leak. But there is also an invisible leaking path. Pressure switch systems rely on a non return valve being fitted to the inlet side of the switch. This is designed to lock the pressure in the pipework. If the NRV has a small leak past its seal, the water will escape back along the inlet pipe and thus be invisible to the eye.
Further submersible Vs inboard points;
Easy to swap a submersible pump.
Easy to steal or lose a submersible.
Can't lose or forget an inboard pump.
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