You could have an in-line fuel filter which is common on 1988 P-30 chassis. This fuel filter should be located on the right or left frame rail of your RV (depending on which side your fuel line runs from the engine compartment). Sorry I could not be more specific than that. You are going to just have to start tracing the fuel line from the carburetor until you finally run into the filter.
To make it even more complicated, some RV owners have taken it upon themselves to add an additional in-line filter. So once you find the first one keep following the fuel line to make sure that you do not have an additional filter. If all else fails take it in and have a mechanic trace the lines and locate the filters for you.
Everything-About-RVing.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com.