1970 Winnebago trailer, model 215 tires and tire pressure

[carlh]

I have a 1970 Model 215 travel trailer, but I'm not sure it has the correct tires on it.  Even if they are the correct tires, I don't know the recommended tire pressure.  Can anyone help me out?
Tires currently on the trailer are F78-14s, load range C.  The local RV repair shop suggested I pump them up to 50 psi, since that is the maximum pressure indicated on the sidewall of the tire.  That seemed like bad advice, so I have used 45 psi, but it appears the middle of the tread is wearing out faster than the sides, so I think 45 psi is probably still too much.  It has also occurred to me that load range C or even the size of the current tires might not be correct.  Does anyone know the correct info or where to get it?  I've asked Winnebago, but was told, "Winnebago never made a Model 215", even though I can read it very clearly on my trailer. 
Thanks for any help you can provide!

[eXodus]

Put it on a CAT Scale.

Most tire manufacturer have a chart where you can adjust your pressure to the load you are carrying.