Here's a 1974/1975 Dodge Class A Motorhome wiring diagram. (use your fingers to "unpinch it" larger).

Hopefully your Travco is similar.

So after getting cross-eyed from looking at that pile of spaghetti, I decided to highlight the starter circuit.

The starter relay pinouts on the diagram are "sketchy" at best so here's a pinout of a 1975 vintage Dodge starter relay.

It matches up pretty good with the 1975 wiring diagram so maybe yours is the same.

So, using my wiring diagram colors, not actual wire colors (you can check if the wire color abbreviations on the diagram match yours):

In the diagram, the (red) heavy gauge wires going from the battery to the starter relay and down from the starter relay to the starter motor are connected to the threaded lug labeled Battery Terminal.

The thin (red) wire going down from the starter relay to the starter solenoid is connected at the terminal labeled Starter Solenoid Terminal.

The thin (black) ground wire going from the starter relay down to the neutral safety switch is connected at the terminal labeled Ground Terminal.

The (pink) wire from the Start Position of the Ignition Switch is connected at the starter relay terminal labeled Ignition Terminal.

So, I'd probably just take a shortcut and grab a screwdriver and jump the threaded "Battery Terminal" lug to the Starter Solenoid Terminal".

If the starter turned I'd know my battery, battery cables and connections, the wire from the starter relay to the starter solenoid, and my starter and solenoid were all good.

I could turn my attention to the "control side" of the starter relay's connections.

The starter relay's coil gets its Ground from the Neutral Safety Switch, so with the Travco in Park grab a test light and check for Ground at the starter relay terminal labeled "Ground Terminal.

Next get a helper and have them hold the Key in Start as you test for battery voltage (12 volts + or -) at the starter relay terminal labeled Ignition Terminal.

If you aren't getting battery voltage there, scroll right in the wiring diagram and make sure the ignition switch is getting power from that 20 amp fuse.

If everything is good replace the starter relay.

It doesn't matter if it's a motorhome or a motorcycle, or if there's multiple circuit failures.

Pick 1 circuit and troubleshoot that circuit. Chances are when you find the problem in that circuit, it's what's affecting the other circuits.

Well I really don't work on motorhomes but basically follow the power from the battery back to the switch make sure the switch is getting power if it isn't you can always rewire the power to the switch if the power is getting switch and no power comes out replace the switch