With the engine shut off how much free play is there in the steering wheel? If it's more than a half inch this could be your problem. Unfortunately Monaco did not use the TRW steering gearbox in all of their motorhomes... you can adjust the free play out of the TRW but not the other manufacture. If the Pitman arm has a large boat on the circumference holding it on the splined shaft of the gearbox it's a TRW. On the side is a jam nut and adjusting screw that will take the excessive free play out of it.
Assuming the tow in is correct and you have less than a half inch of free play then the problem may be the nut behind the steering wheel not giving steering inputs long enough to take affect... much the same as oversteering a boat that is slow to take steering inputs. You have to give smaller inputs longer to take affect or you'll be chasing the motorhome and all over the highway. On a good highway with no wind see if you can keep it between the lines by cutting your steering movements in half. With practice you can anticipate what you need to do... going under a bridge when you have a crosswind.
*I have 80,000 miles on my Monaco and it's a pleasure to drive. Other than adjusting the gearbox early on, the rest of my front end is stock and other than checking the toe end myself it has never been aligned since leaving the factory. If your tires are wearing evenly with 20,000 miles on them there is no reason to do an alignment.
