As mentioned, the 3.0 is the weak link in the driveline, followed by the transmission, especially if the owner tried to tow anything. In the motorhome the engine is working much harder than in the pick-up and performing routine maintenance is essential. The biggie is valve adjustment. As the valves wear they tighten up until there is no clearance and you get a burned #6 exhaust valve, quickly followed by others.
The bad news.... Valve adjustment uses shims and is about a $600 job and often is overlooked or done wrong. At 135k that is a high mileage engine and, without a lot of service records I would look elsewhere.
That said, Sunraders are the primo of all Toyota Motorhomes and nice ones with the V6 have sold
as high as $18,000+ in the past year. But to repeat what Pete said, the VZ-E 3.0 V6 is Toyotas most problematic engine.
PM me for more info if desired.