Finding a reliable inspector is a function of your ability to evaluate the inspector and the expected process.
To enable the inspector to do a more complete inspection, the inspector will ask the buyer/client to talk to the seller/non client in this instance, to have the RV relocated to a local campground for the inspection ,and fully activated for the inspection, and for the seller to be available to open and then secure the rv when finished.
It is best to not have the inspector interact with the seller without you knowledge and permission and consideration as to what that means, as it sets up a potential negative interaction as the seller may want to hover around the inspector and/or attempt to ascertain his findings...and prevent the inspector from focusing on the inspection.
Such findings should be confidential for the buyer as it may be brought into the negotiations and part of the buy/no buy decision.
Some sellers do this in advance as part of their marketing of the condition the RV is in.
The inspection is likely to be more that what you would get with a PDI like dealer inspection and you may get a written report and pictures as part of the inspection.
Depending on the inspector, some inspectors offer three fixed types of inspections, other inspectors work with the seller to customize the inspection.
The inspector may take a variety of fluid samples, but this should not replace, but complement an examination by a mechanic and chassis tech.
Your knowledge of what is important or your willingness to spend some time with the inspector or to study the subject to flush what you want done and can/should be done, is important. The training and experience of each inspector will vary and so will the end product vary and cost.
If you want a McDonald's type inspection you can find that, but customization and time adds up if you want a customized service or you want more interactions or to be present at the inspection so you can talk through the process with the inspector without needing a written report. Some of the written reports will have a couple of key issues noted and the rest is just cya filler material to convince the buyer they did what was asked, since the buyer was not present. Something to think about.
Because of legal liabilities and potential to break things, inspectors are being pushed to limit themselves to visual inspections...even to the point of not moving anything.
Note: I'm an independent Certified RV Inspector and mobile technician, but I'm not close to you. Best of luck.