Not looked a one in a while on an RV, so consider this as "general" things to look for more than giving any specific spot for the chassis your RV is built on?
I think you can get this one done without going too far into the controls or under the dash in many cases! YEAH!
There is a resister pack at some point in the airflow. That small pack has a set of wire coils which make varios resistance, depending on which setting the control is on. Those wire coils change how much resistance the blower circuit has as the switch is changed. They also tend to get hot and are placed in the airflow to cool them but they do tend to burn out with age.
So when looking under the dash and depending on different chassis it may be out under the hood, but expect to see a big bulky cover over the space where the heat and cooling is done.
This is where things can be build different and I see the parts drawings for your RV show the pack as being outside under the hood where the heater and air coils, etc. stick through the firewall! I have normally found the pack inside near where the back of the blower motor sticks out??
So it may take a bit of looking, depending what your chassis did. Can you see up under a flap often set in to conceal the motor, etc. that is almost directly over where the passenger feet go? I have found the pack inside but your drawing shows outside??
View attachment 2489414
Idea is to spot andelctrical plug for 4-5 wires that plug onto the part of the pack that extends out of the cover. Not too many sets of wires on that part of the RV but just maybe less than 2X2?
I think of it as inside and near the motor but not looking at yurRV, so keep an open mind! Maybe worth making sure it is pluggged good but theser are kind of common to burn one or mor of the coils and maybe two screws to hold it in, making it not too hard to changed? But that is also depending on how they built it!!!
Common auto parts store item for better price/availability than RV supply!
This is a common one for Ford:
View attachment 2489417