P-metric tires are passenger car tires. They should NOT be used on a motorhome. They are not made to handle those kinds of loads. What load rating are they? Instead of a load range letter, on the sidewall you will find two or three numbers followed by a letter, i.e. 98T. You should absolutely be using light truck (LT) tires.
Whoever put P tires on your motorhome was obviously trying to buy tires on the cheap. You were wise not to try driving with only a single rear tire. What really surprises me is that anyone even makes an 85 series P tire. That is an aspect ratio usually used only for truck tires. The closest I'm aware of is 80 series, and those are very unusual.
LT235/85R16 load range E is probably the most common tire size for that size chassis. Your RV very likely came new with either LT235/85R16s or 7.50-16s. A 245/75 will probably not work in the dual positions. The sidewalls will rub, leading to tire failure.
I would strongly recommend getting those passenger car tires off as soon as possible. The next time one fails you may not be so lucky. In the meantime, make sure they are inflated to the highest pressure you can safely use.
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79 27' Holiday Rambler Statesman, 78 32' HR Imperial, 85 36' HR Imperial 5th Wheel