There is a standard trouble shooting chart for this kind of thing
Flush the cooling system.. make sure the radiator is not clogged up with crud.
Make sure you have good air flow
Make sure engine is properly "Adjusted" (Fuel/air ratios and things like that)
If it is a side mounted radiator then that is probably full of engine oil blow by. You will need to take it into a shop to have the oil removed from the radiator. Last time i got that done it cost me $250.
This happened to me when I when I was in Wyoming last year in my MH with a Cat 350. The radiator was plugged from the blow-by with oil and dirt on the back side of the radiator. Took it to a Cat shop there and they steam cleaned the Rad. and added a 2" hose from the blow-by tube to the rear of the MH (about 8' of it). If you look at the back side of your radiator you may see the same problem. It may add a few spots of oil to your towed vehicle but anything is better then a plugged radiator !!!
1 - Buy an infrared temperature gauge from Harbor Freight for like $10.
2 - Point it at the radiator hose fitting (metal part) at the top of the engine. This will give you engine outlet temp. Compare to gauge and rule out gauge problem.
3 - Point the gauge at the inlet fitting at the top of the radiator (metal part) - this is the inlet temp to the radiator.
4 - Point the gauge at the outlet fitting at the bottom of the radiator (metal part) - this is the outlet temp to the engine. The temperature drop should be anywhere from 30-60*F. If you are not getting this kind of temp drop then you have a radiator cooling problem.
I could speculate at top issues but we need to know if this is really overheating or not. Here is about the "most" to "least" likely...
1 - Cooling system contamination - flush and fill never happening, sludge build up - low temp drop in radiator
2 - Bad thermostat - probable direct overheat/overflow
3 - Electric fans not cycling - low temp drop in radiator
4 - Head gasket or head failure - oil in water or water in oil
5 - Other engine failure
6 - Some change to radiator/engine shrouding that changed airflow through radiator.