Mine is on a little aluminum 'sticker' on the left side of the tongue. The propane tank holder on my FunChaser is welded on, I doubt if there's a number under there.
we are the ones that found it under the propane tank mount. Although I think that mount must have been moved because the battery is outside. Where it is on the tongue is curb side rail (top) about 22" from where the center of the ball would go on the hitch. Ours was so pitted that the inspector OKed it but would not register till we get another vin tag mounted from the state (OH). Ohboy another couple weeks till we can camp. Hope yours is in better shape Peggy