Depends on where you are, ambient temperatures, and how much sunlight. Also depends on what you mean by "cool."
My 2004 Itasca Spirit, with similar insulation and window area to the Sightseer of the same era, will reach 100 F inside in full sunlight within two hours on a 80 F day. My single 15K BTU/hour A/C will bring it down into the 80s within an hour, but it is faster to open the windows and draw in that "cool" 80 F air before starting the A/C.
In full sun on 100+ days, I can expect to cool down to the mid 80s in sunlight, somewhat cooler in the shade. At night, if it is still 100 F, I can get the temperatures down into the 60s if I want to, but I don't want to. I don't cool my home much lower than 80 F when temperatures are in the 90s or 100s.