The hydraulic reservoir looks identical to my coach also (3 filters stacked). I think the original might have been NELSON 84101B but looks like those are not available anywhere.
I found this as a possible replacement for almost 1/3 the cost of the Napa filter: 84101B: Fleetguard PSR ELEMENT $11.64 (https://shop.moderngroup.com/84101b-psr-element/
). Does anybody know if that would work as there are no dimensions.