[JRTJH]

Your idea of spraying a protective coating on the underside of the trailer will protect it from about 50% of the risk. The other 50% comes from above the floor (roof leaks, window leaks, side vent/molding leaks, plumbing leaks and "just plain bad luck leaks"... Your idea won't protect you from that half of the problem, but it would add significant weight, which in a boat might be considered "well spent ballast" but in an RV, is extra, unwanted weight that detracts from payload, adds to tire wear and increases demand on the tow vehicle.

Then, to add insult to injury, if you should have a problem with a rotted/damaged floor, that Rhino-liner will make repairs even more difficult.

My vote: No, it probably won't be cost effective and may not prevent the damage you're trying to avoid.