rynosback, I have an external dual filter setup on our trailer and buy the filter replacements at Lowes (Home Depot also carries them). They come in a two pack so I always have spares. These are 10" filters and I bought the filter setup from The RV Water Filter Store. Also bought our Watts pressure regulator from them. Hope this helps.
Internal coach filter has been a 5 micron carbon block filter. I've bought at Home Depot, Lowe's and Amazon.
Now using an external triple filter system from Water Filter Store that has a sediment filter, a carbon block filter and a BIRM filter for heavy iron in the water in Michigan. These filters are .5 micron and 1 micron filters and adversely effect water pressure so I'll probably go back to 5 micron filters on re-order.