In most RV tires there is no difference between front and rear axles. They are designated as "all position" tires. You could use different tires if you chose to, but most do not.
You may have trouble finding an 8R19.5 size - it is not widely used anymore. Goodyear lists the 225/70R19.5 as a substitute for the 8R19.5. It is about the same load capacity and circumference, but requires a bit more dual spacing and is a little bit wider, so make sure you have clearance for it if you go with that size. There is a conversion checklist here:
http://www.goodyear.com/rv/general/conversionchart.html
I'd call around the truck tire places for prices - there is often a lot of variation. Several manufacturers make tires in that size range, so see what they have. I've had good results from GCR tires in Florida and they have a Houston location as well. Give them a call for a price.