Can a valve be replaced without removing engine on 1992 Ford E-350.

What is your question?

My Ford E-350 carries a Tioga Motor home. It sits for long periods, but I do exercise engine every 1-2 months and put fuel stabilizer in the fuel tank. Previously, ran fine getting 8 mpg with plenty of passing power towing Jeep Wrangler to my present RV park for the last 7 months.. But upon latest startup engine began backfires sporadically through one exhaust port when engine is revved 1,500 + rpm. Idles a little rough indicating one cylinder not firing consistently with full efficiency. It looks very tight under the hood, but would it be possible/feasible to remove the head(s) with the engine in place to replace the valve?

What makes this problem better or worse?

Worse when engine accelerates to high RPM.

How long has your 1991 Ford E-350 Econoline had this problem?