Heat gauge

A couple of general recommendations.  I don't have direct experience with Dodge Sprinter/MB vans, but cooling system problems in heavy duty and utility vehicles is pretty common.  First, the tech that did the flush was going after low hanging fruit.  If the coolant in the system hadn't been maintained or changed between when the vehicle was new and now, it is possible that the radiator is full of gunk, so the flush is a way to try to address that, but a severely clogged radiator isn't going to be fixed with a typical system flush.  Manufacturers also tend to design cooling systems around "best case scenario" situations, effectively designing cooling systems that are sufficient... just sufficient... for a new vehicle running brand new coolant, and always being serviced according to their schedule.  There is this joke out there, the difference between a German vehicle and a Japanese one is that the German vehicle is just as reliable as the Japanese car as long as you follow the maintenance schedule exactly as the Germans intend.  If you don't they become unreliable and turn into junk.  The Japanese car will give you 500k of service if you follow the maintenance schedule, but they design in the human factor, ignoring the maintenance schedule, so that means the vehicle will run great for the first 250k, with minimal maintenance and then kind of become loosey goosey for the last 250k, but otherwise will just soldier on.

That said, I think your cooling system or rather the radiator may be clogged.  At that point your only solution may be a new radiator.  The good news is that there are plenty of non-oem manufacturers that have upgraded radiators for your application.  It is pretty common for a factory radiator to be just a 2 core radiator.  If you find that replacing the radiator is a viable solution, you'll want to source a 3 core radiator, or any radiator which is at least one core more than the number of cores in your stock radiator.  The extra core will make a huge difference in cooling performance.

I have an old Bounder on a Ford chassis.  Ford is notorious for installing 2 core radiators when a 3 core would perform better.  My radiator sprung some pinhole leaks and needed to be replaced.  The mechanic sourced a 3 core radiator and I have zero issues with overheating, even on the hottest days with the dash AC on.  While he was doing the radiator swap, there was plenty of space to work on front accessories, so they replaced the water pump and the serpentine idler, all wear items which were near their end of life.  You may want to see if that is something that can be done while the radiator is out as that is good preventative maintenance.

Adding electric fans to try to move more air through the radiator is a common recommendation with DIY crowd, and sometimes works, but the real issue is providing your engine coolant with sufficient surface area to dissipate heat.  The more cores you have in a radiator, the better it is at dissipating heat and keeping an engine cool.

You may have a transmission cooler in front of the radiator also. You need more airflow over the cooling fines on both the condenser and radiator. If you have an electric fan, see if there is room to install one more. It can go in front on the condenser, just turn it so it blows in. If it is a mechanical fan (clutch type) look into a fan with a couple more blades

running the ac makes the engine work harder so if it is prone to overheating it will, the reason for it over hearing is what you need to find, weather it be somthing like what way2roll mentioned, or somthing else.  another comon cause of being close to that edge if everthing else is good, is running to hot of a spark plug.  so if you did the plugs and they didn't have the right one and you put a hotter one in it then that can be an issue also.  but realy anything that can make the engine wirk harder can contribute to that.  a idler pully thats starting to go, or a berring in an altanator and do on..  

Welcome. Overheating with the AC on sounds like a problem with the condenser fan. The condenser fan is responsible for cooling the air conditioning condenser, which heats up when the AC is on. If the condenser fan is not working properly, the condenser will overheat, and this can cause the engine to overheat as well. I think it was a fairly common problem on the Dodge and MB sprinters a while back.