Matt,
I think your 98 Horizon will be similar to our 97 Sundowner. If so, there will be a fiberglass motor box that the rear bench seat cushions sit on. Once the cushions are removed, the box can be removed by removing a few screws that secure it to the floor and the rear bulkhead area. Once the motor box is removed, the oil filter is much more accessible.
Use a cut open anti-freeze bottle (engine anti-freeze, not a round RV anti-freeze bottle) to drop the oil filter into and to catch the oil.
Here's a photo with the box removed:

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Gordon Arnold
New Hampshire
2003 268 Vista ..................................................................Prior: 97 245 Sundowner
