I have also used flexseal on the roof of campers with great success. Was a fair amount of prep work but well worth it. I also just picked up an older PUP and although it doesn’t leak I plan to do it for piece of mind.
Be very careful without a regulator. Keep the pressure very low. Pressure regulators are super cheap and can be found at Walmart. I would rather just keep that on than worry about blowing my lines out and flooding the camper.