Welcome to the forums, lots of great info here.
Smart to get the initial work done in a good shop that is used to dealing with coaches. As you say, you will then know the location, size, etc. of all filters. Even though the age of the coach would indicate that all is out of warranty you can still take advantage of the component manufacturers info, either on the web or hard copy from them.
"Home is where we park it" SEMPER FI
Winnie 30 RESS UltraLite TT 2015
Ford F250 XLT 2015