So I see a lot of money being wasted by changing oil too early, along with oil filter.
Why don't we change this philosophy?
Well, it depends.
Despite the silly price of some oils, saving maybe £40 by extending the oil change interval 50% above the recommendation is probably endangering the life of the engine.
The recommended oil and filter change interval is set by the maufacturer.
They have nothing to gain by over-frequent changes, and they have something to lose. A longer advertised service interval is a marketing advantage.
The maker sets the interval so that under normal use, the engine will last until at least the end of the warranty period.
A motorhome is not used in a normal way, and certaimly not an optimum way (frequent long journeys at light load but fully warmed up). They stand a long time between trips and always travel nearly fully loaded.
They tend to be kept far, far longer than the equivalent van, several years (or even decades) after the warranty expires.
Oil degrades several ways. In diesel engines it can absorb carbon deposits and become gummy. It can have its long chain polymers chopped up and allow metal-on-metal friction. It can absorb water or other contaminants, but that's not normally an issue.
Generally, it is wise to service a motorhome at somewhere between 50% and 75% of the maker's recommended service interval.
Yes, it costs a little bit more money but it is a sensible investment.