I gather the camper was "new to you".
How about this as a scenario:
Perhaps the previous owner did what I did when I replaced my OEM C rated tires (50 PSI max) with upgraded load range D rated tires (65 PSI max). Since the C rated tires were minimally sufficient to carry the rated load, I took the best of the bunch (the OEM spare that was only used to drive the camper about 100 miles home when I blew a tire in DC) and put it back on the spare tire mount. Leaving 4 D rated on the camper and a C rated spare.
THEN, he runs over a nail with the new D rated tire and puts the C rated spare on to get home. Lazy, he does not put the repaired flat on the camper but leaves the C tire on the axle. Then, he decides to sell the camper with the mismatched tires to unsuspecting you.
He may even have used the camper like that for a while before selling it.
Out of balance and mismatched load rating could easily have destroyed that tire. Time to buy a tire identical to the 4 on the camper to use as a spare.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW