Probably the original 13" yust fitt under the RV and a larger rimm would mean that you have to use a lower Aspect Ratio (further AR = hight/width division)to get the same diameter.
This thoug is not good for the contact to the road of the tires and they probably need higher adviced pressure.
Might even that you have to compensate the lower maximum load with a wider tire and so rimm, wich will have to fitt, wich makes it need even a lower AR.
So my advice is leave it the way it is, but if you do , use my Traveltrailer-tirepressure-calculator or give details like GVWR and GAWR and configuration ( numer of axles and total tires on the road.
From chosen tires the maximum load and pressure needed for that , wich is not the maximum pressure of tire, but also give the sises so I can yudge howmuch to substract from the maximum load in case of low AR.
If you want to play with the calculator yourselfes, click this link to go to a map on my skydrive that belongs to my hotmail adress with same username as here. Then CLICK RIGHT
on the needed spreadsheet and choose Download
. Dont leftclick or choose open in web-app , goes wrong , and also dont use open in Excell, is to complicated and asks to log in with your hotmail or whatever adress.
After download and eventual virus-check ( you never know what those foreigners put into it
) open it in Excell on your computer, but Open Office Calc can handle it too.
If you fill in the tires maximum load and pressure, and the GVWR is given, a warning is given if the tires have to less maximum load or poor reserve, so you can compare.
Much good for 2014 from Holland
Edit its 10:20 here in Holland at the moment but I see its 01:20 there so middle of the night.