There were two reasons for looking into this topic. The two plastic tanks are slowly going blind and I wanted a history of the heating oil tank level in Home Assistant.
I found some useful inormation @Tecson homepage. As my two battery tanks almost always have the same level, I decided against the variant for battery tanks and ordered the e-litro net instead, which I integrated into our home network via cable. I use the measurements of the single tank to calculate the total level in Home Assistant, which is then displayed as the current level in the app.
The installation of the tecson sensor is well described and if you still have a free “blind cap” on the tank, you can simply use it to bring the sensor into the tank. Alternatively, you have to drill a hole. The tecson support team advised me exactly where to drill the hole. I sent them photos of my tank system by e-mail.
As soon as you have integrated the tank sensor into your local network, you can access it via http://IP-adress. For the integration with Home Assistant you shall use the XML interface via http://IP-adress/xml
I have added following in my configuration yaml:
In the section “rest:”
- resource: http://192.xxxx/xml
scan_interval: 3600
sensor:
- name: "Oil tank 1"
device_class: volume_storage
state_class: measurement
unit_of_measurement: "L"
unique_id: oellevel_raw
value_template: "{{ value_json.seqSmartbox.seqData.seqTanks.seqTankLevel.tankLevel }}"
This is the configuration of a sensor for a single tank. As I have two tanks of the same size, but only measure in one tank, I have added the following to the template section.
- name: "tank level"
unit_of_measurement: "L"
state_class: "measurement"
unique_id: oellevel_current_sum
state: >
{% if states('sensor.oeltank_1') | int(-1) >= 0 %}
{{ states('sensor.oeltank_1') | int * 2 }}
{% endif %}
The if statement checks whether the current value of the rest sensor is a number. Only then will the value be used for the calculation. For example, if you have 4 tanks of the same size, you must of course insert * 4 
Info: Link to the german version: it-und-hausautomation