First thing you should try and do is verify that in fact your fuel system was vandalized. How do they (dealership) know that roundup was in your fuel tank? Did they send the fuel sample out to be tested? If so, did you get a copy of the report on the fuel sample?
What model Deere is it? A D series or earlier? 2005 to 2009 was a mechanical fuel injection system, 2010 D series are electronic fuel injection system. Not that I can tell you anything other then that.
There are two fuel filters on these machines, a primary in line element just on top of the fuel tank and a water separator fuel filter towards the upper back of the engine. The primary first filter is 99.9% of our fuel issues. It's doing it's job collecting trash but it clogs fairly fast with crappy fuel and starves the machine out of fuel making the operator think the unit is running out of fuel.
The D series machines will have a message prompt on the dash telling the operator the engine is running at X % derating and dropping. If an air filter or fuel filter start to get clogged and the computer senses this, it will prompt the operator the unit is not running at max proficiency.
My suggestion is to take it to another dealership if you think you're getting jerked around for a different opinion to see what is going on. Demand all service records from your dealership. You can call a Deere rep to help get involved with the issues to try and help as well.
Keep this in mind though, They are not going to buy your machine back. First they will get to the bottom of your problem if possible. If it was vandalism, they will fix the unit but they will charge you for that repair. If it's the machines fault, then you'll have to take that up with your dealership and Deere rep. Some of that depends on if it's out of warranty and how long ago etc etc. I can not tell you what they will do or not do as I really have no idea and don't really understand whats wrong with your unit.
If you have sabotage happening then you very well may have other equipment at risk as well, including your loner unit.
All I can tell you is do your homework first before pointing the blame.
I'm only allowed one thought a day