No problem. My system has security which is why the code has preferences for dev id and key for access but yours being open should be even easier.
My system returns json for example
{“system_id”:432864,“modules”:30,“size_w”:7500,“current_power”:1829,“energy_today”:1377,“energy_lifetime”:9223891,“summary_date”:“2015-05-18”,“source”:“microinverters”,“status”:“normal”,“operational_at”:1409425500,“last_report_at”:1431972708}
I am not really sure what you will have to do to process the XML instead of the json. Perhaps the methods they have provided will detect and support either.
The docs for the response part are found here.
http://docs.smartthings.com/en/latest/smartapp-developers-guide/calling-web-services-in-smartapps.html
Understood about learning language from docs but I can tell you I have never programmed in groovy either. But I am a current java developer so that helps a great deal. The 20year gap may make things a little more difficult 
If the processing isn’t done for you perhaps you can use this to convert from XML to json
I dont’ know if JSONObject is available in ST ide. You can’t import new packages into the ide, you get what they provide you only.
Perhaps someone else can explain if ST can handle XML results.