Overheating - only when towing

[Buddie_37]

Hello fellow Powerstroke enthusiasts! I'm fairly new to the forum but not new to the Powerstroke scene. I apologize in advance for the long post but I would like to include as much information as possible. I have a 2003 6.0 with close to 180,000 on the odometer. I've owned it since 2006 and when I bought it, it had just over 100,000 miles. I haven't had any issues with it other than a recalled ICP sensor and a bad transmission solenoid. It is pretty much stock except for a Bully Dog RFI cold air intake and 3" turbo-back exhaust (no cat). No tuners or programmers at all. Last summer after researching multiple posts we decided to flush the cooling system and also install a coolant filter. I purchased the filter kit from dfuser and used the OEM Ford Gold coolant to do the flush. We have only made two trips towing anything since the filter install and they were in the winter months but didn't have any issues. Yesterday morning we hooked up the 26 ft toy hauler and headed to the mountains. We left about 9:00 AM and live in AZ so it was already in the mid 80's. I usually take it pretty easy towing and the first couple small hills were no problem. The first long haul the cooling fan kicked into high gear but the temp gauge slowly started to climb up and got high enough to cause the check gauge light to come on. By this time we had reached the top of the hill and once we started downhill it cooled right back down immediately. It did this again on the next two hills so about 20 miles outside of town we decided not to push it anymore and headed back home. Keep in mind the toy hauler was completely EMPTY (no water, no toys). I'm not sure where to even begin. There is no coolant being lost so I don't see how it could be a head gasket issue but I suppose it could be a possibility *cringe*. It has a Bully Dog Outlook Monitor installed but it provides such little information it's really no help at all. I think purchasing a different monitor will be one of my next steps. I've seen some related posts about it possibly being the belt tensioner, radiator, water pump, EGR cooler, thermostats, and a few others. Any suggestions? Also, at what temp does the Check Gauge light come on? It wasn't puking any coolant and the temp gauge hadn't hit the red yet but it was pretty darn close. Thanks!