I make a living in the Home Inspection business and I'm always interested in watching the integration of inspections into RV purchases. I'm going to go out on a limb and guess the $1300 inspection that "includes looking for water intrusion" uses a thermal imaging camera. If that's the case a good chunk of the money you are paying is for the equipment used.
While thermal imaging has its place, it's common in the Home Inspection industry for a guy to miss major things elsewhere while burying his face behind the expensive camera so beware of that.
Home Inspections in most states have licensing, Standards of Practice and years of consumer vs. inspector disputes to learn from. RV inspections? Probably not the same. I'd be curious to see the contract the RV inspector uses. In the residential industry all contracts will have a limitation of liability clause (where they are legal) that limits the inspector's exposure to the fee paid. I'd expect something similar in the RV inspection but with the industry being so new it's hard to say.
Knowing what I know after 15+ years of inspecting something worth a lot of $$, if I were looking around for an RV inspection I'd probably just try to spend a lot of time on the phone or email to get a good feeling about the guy doing the inspection. Most importantly..... and this is CRITICAL. Read the fine print of the contract and anything else the inspector puts in front of you. Far too many people assume things and it leads to huge headaches later. Trust me, the headaches are on both sides. Make sure you are 100% sure of what you are getting for your money.