Once you given the needed data , I will put them in my made calculator and give picture of it in my answering post, with extra explanation.
Need best seperate wheel loads,2th best axle weight of vehicle in vacation loading, 3th best GAWR's and GVWR.
configuration , given already by 6 tire is or rear 1 axle dual or 2 axles single load.
And maximum speed you wont go over for a minute in your use.
From tyres : Maximum load or loadindex.
Kind of tire to determine the AT-pressure ( wich is not the maximum pressure of tyre), already given by AT-pressure of 125psi is called H- or J-load.
Speedcode of tire ( In Europe always given in letter J to R, most larger tires N ( 87m/h) or L ( 75m/h).
But if you are not shure give sises on sidewall given and as much information you think is usefull, then I will google for it. Best is to give the sises anyway, because in the larger tires there is much wrong conversion that is later corrected.
EDIT: googled your berkshire and saw picture with 1 dual axle behind.
The Pressure/loadcapacity lists given for these large truck tires mostly give 75m/h as maximum speed of tire.