Then hook up the jumper cables again from your running car's battery and hook them to the terminals of the battery in your XF. Crank and fire the XF and if it keeps running and running for an hour then that leads to an alternator issue and eliminates bad grounds, etc. I assume you don't have a multimeter that could tell right away if your XF alternator is delivering charge that is making its way to the battery while running so this is one easy test.

Be warned, when you state that you are just gonna have it towed to the shop and let them fix it. That is gonna cost a fortune as Mercedes won't touch it and Chrysler may try and run you down a rabbit hole of $ as the techs around today have no clue about how to work on Mercedes. That leaves independent euro shops who know these old SLK 320's and can work on them and they are not cheap. Maybe you have that kind of $ and maybe you don't care. If so, good for you and have at it but for those who think they want to own one of these cars and have no tools or a clue how to maintain them they become deep, deep money pits. Just my 2 cents for what it may be worth.