For one thing, if you are measuring 13.8V, that means your converter is on so you will measure battery voltage on the load side of the switch. You are likely measuring 13.8V on the battery side of the switch since it has a surface charge. Disconnect shorepower, turn off the switch and measure again.
If you actually need to squirt wd-40 into a switch to make it work, time to buy a new switch.
2017 Coachmen 233RBS
2018 Ford F150 Lariat 3.5 Eco
"Common sense is not very common"