Motor oil weight

1988 would be back in the era where manufacturers were told by the EPA that if they wanted to use low viscosity oil in the EPA fuel economy tests, they had to recommend that same viscosity to the consumer.

So that is when 5W-30 became the recommendation. I'm not saying it is bad. I now own a car that takes 0W-20. But I am saying that IMO unless you operate your 1988 motorhome in temps lower than 10 or 20 degrees F, you could certainly go with the 15W-40, or 10W-30. You'll do no harm and maybe some good, given the heavy MH load, in warm weather.

I would pick manufacturer's specification over "hood scribblings" any day.

Id use what's marked on the oil filler cap, the owners manual, or what Ford says to use. Run it for a while and if you think you're having an oil issue then seek professional advice.

Mine says 5-20. using that, I had to add oil every 2000 miles or so. Moved to 10-30...no oil loss between changes. (every 4000 miles) I use a "blend". The advantage to a "blend" is that should you need to add oil (like on the road) you can add either syn or fossil. Won't harm anything.
(However..if you have ALL syn or ALL fossil, you have to stick with that should you need to add )

Most vehicles have the oil type marked on the oil cap.

Use what the manufacturer recommends. Synthetic oil is the best choice.
Motor Oil Viscosity