That filter was one of a defective batch made by Onan and it is leaking around the seams. Onan does not make that filter anymore. They make an adapter to take the place of the filter. Buy the adapter and removd the hose off of the filter and then remove/unscrew the old filter and screw in the adapter, slip the hose back on and you should be good to go. There is a second filter screwed into the fuel pump and it should be replaced with the new style filter. I will give you a link to the adapter and the new filter part number as soon as I find it. So in essence you end up with only one filter instead of two. I had the same leaking problem and I used the adapter.
Onan adapterThe old discontinued filter was Part number 0149-2629, The New replacement filter is part number 0149-2341-01. Here is the link
New Onan filterI would try Channel lock pliers to remove/unscrew the old filter