If the relays were added by the body manufacturer, I doubt if they use intermittent duty solenoids. If the loads are the same you can replace an intermittent with a continuous, not the other way around.
I want to replace all the solenoids and circuit breakers in the panel as well as the engine bay as there are a few more in there.
I question why you feel the need to replace all of your solenoids? They rarely go bad. At most I might carry a spare or two. Circuit breakers don't wear out and for that matter are rarely even tripped. In 50 or so years of working on all types of vehicles, about half of that for a living doing electrical troubleshooting, I have seen very few solenoid failures, most of those were the old Ford fender mounted starter ones that carried a heavy load. I think one maybe two bad circuit breakers in all of that time.
If your bus came with a lift of some sort, there are all kinds of switches and relays so that the bus can not be started unless that lift is fully retracted. Many also have switches so that the lift can not be deployed unless things aren't "Just so". Many had to even have the parking brake engaged.
When trouble shooting a no start, get down to just the basics needed to run, per the chassis manufacturer wiring diagram, in your case an '89 E350. If you aren't going to use all of that other stuff just eliminate it.