Any shunt that develops a maximum voltage drop of 80mV (if used to sense current in both directions) or 160mV (if used to sense current in one direction only) and supports the maximum current you want to measure would work.

Here is one we sell and it fits the PowerMon device like a glove:

500A/75mV/0.5%

Make sure the location services are enabled. Also make sure that the app has access to Location Services (under Application Manager you can control what services is any application allowed to use). Also, if you just upgraded to Android 8 (Oreo), restart your phone. 

Android 4.4

Any relay with a coil voltage of 12V and current less than 0.5A works. 

It is better to use a solid state relay (TL-SSR12100DCU) but if you insist on a mechanical one, here are some examples of automotive relays: 40A relay, 200A relay

Also, note that a relay is only required if you are planning to use the low voltage disconnect or any auto-disconnect feature or the remote ON/OFF feature. For an RV / boat installation it is recommended to use a relay. This will allow you to turn the power off entirely when in storage (to avoid parasite loads draining the battery). The low voltage disconnect will protect the battery against over-discharge which can damage it. 

Yes, you can use an SSR.

One of the advantages compared to a mechanical relay is that they require a much lower current to hold in the ON state (about 20mA compared with up to 0.5A for mechanical relays). This makes a big difference in boats and RVs where power is limited. Also the contacts don’t wear out, there is no sparking and it is quiet. 

The TL-SSR12100DCU SSR was built by Thornwave Labs specifically for RVs, boats and other vehicles with a 12 Volts electric system.

PowerMon allows you you install the shunt on either side, or even in between batteries that are connected in series. It doesn’t matter as long as the maximum voltage on the shunt is kept within -4V to +65V. 

We advice to install it on the positive side because this way you will not disturb the ground connection. 

We now have data logging with storage onboard the device! PowerMon can store up to 3 years worth of data. This feature is available only for hardware revisions at least 2.30. All battery monitors sold today support data logging. Customers that have a device with hardware revision 2.30 will need to update the firmware to the latest version.

RV Whisper also created an integration with our battery monitor that allows you to log the battery data and access it remotely through the Internet. You can find more details towards the beginning of this page, under “Logging Capabilities & Internet Presence”.

Go to the menu and About. Scroll down and check the "Hardware Revision". Only devices with hardware revision 2.30 and up have hardware support for data logging. All devices sold today are 2.30 or higher.
If you have hardware revision 2.30 or higher, you enabled the data logging under the configuration page but still don't see any data log files you may need to go to About and clear the data log.