Success!
I did the primary filter first. It wasn't horrible, but there was certainly noticeable rust-like particles and crud floating around the bottom of the canister. The filter was visibly dirty but not blocked, but the upcoming 2,000-mile trip would probably have ended with a filter problem if I had left it till after the trip.
Refilled the canister to the brim, re-installed, and then ran the engine at fast idle for a few minutes. Not even a single stumble, so I must have done something right.
Since the primary was not clean, I also changed the secondary filter. It looked pretty good with only a little brown coloring to the bottom of the filter paper. Same procedure with about 10 minutes of fast idle running afterwards. No problems noted.
On drive home to Wisconsin from NJ last fall I did have some noticeable loss of power as the trip progressed. I assumed it was the filters getting clogged, especially since the bus had not been used much over the past decades. Filters were changed in the fall after that trip while the bus was at the shop, and they confirmed that the primary was pretty clogged up. With that history, I thought it was important to change the filters again before heading west in a few weeks even though we haven't driven too many miles since the last filter change.
I have two extra sets of filters packed for the upcoming trip, just in case. Hope to not need them.
After all the various repairs and upgrades I've done on this bus it's strange, but changing the filters seems like an accomplishment. I guess I haven't really done that much of the engine work myself, so hopefully this is the start of getting my hands dirty again.