The steel frame underneath an RV trailer is exposed .
My 21t Komfort has a 6 inch tall steel frame .
It came stock with the crank down spare mounted under the trailer just ahead of the axles . The spare has never been used in 11 years .
An answer for the extra spare tire is probably by taking the GVW weight and then deducting your current actual weight and observing how much reserve you have .
If the reserve is a big number and the frame is significant then you wold be good to go I imagine .
My particular GVW on the axles is 7280 # which is dictated by the load capacity of the stock tires which have an 1820# rating each . In actual practice my trailer axles are about 5400 # or less .
Instead of an extra spare I would spend that money on 4 upgraded LT tires with a load rating of 2271 # + each rather than the 1820# stock tires . And forget the extra spare . Actually that is what I did for my TT and my boat .