The way you get a valuation placed on your own labor is to have the trailer appraised by a respected RV appraiser.
Subtract from the appraised value the price you paid for the trailer and the amount you have spent on parts, supplies and other people's labor. The remainder is how much your labor contributed to the value appraised.
Example:
Appraised value = $9,700
Purchase price = $3,000
Parts, supplies and other people's labor = $8,479
Value of your labor = -$1779
;-)
Al