Assuming your '03 with Cat is similar to my '02 with Cat
There is a plastic SURGE TANK on the right side of the radiator looking towards the front of the RV. Standing behind the RV.
That tank has the traditional 15 PSI radiator cap on it. That's where you pour in 'pink' antifreeze. Or maybe 'red'...I forget. Not green or orange. Check the Cat book. There's just enough room above to be able to get a jug of antifreeze in the right position to pour it in. I always get 50/50 Long Life Cat approved, usually from Napa. Cat antifreeze is special stuff so don't scrimp on it.
Check the level first! That's between those two arrows embossed into the plastic. Hard to see if the tank is all grimy.
Those plastic tanks start weeping after a few years and need to be replaced. If you're a DIY'er, it's not that hard of a job...here's my ad free article about it: Radiator and cooling
And here's a picture of the tank...