GMC G30 RV

Moin people! I had bought a GMC G30 RV, which I have upgraded in just under 3 months, transferred it on his own axle almost 100KM and he drove very well, when following he jumped directly again, now after 3 months he just doesn’t want any more. He moans slightly after gasoline at the start process, already 3x the battery is empty (should anyway a new one in) Since I wanted to drive to the TÜV on Monday to get me a list of defects (or f Is this now rather modest that the good piece doesn’t want to start anymore… What can I test first? Unfortunately, I don’t have much idea with US vehicles, you can’t really get on the engine, because it sits more or less under the cab (which can’t be tilted ) Would I be happy about tips.