Lippert doc on which fluid to use: https://lci-support-doc.s3.amazonaws...cd_0002088.pdf
I only have their jacks because my slides are electric. In case you're not aware, the reservoir should only have fluid added when the jacks are up and the hydraulic slides, if any, are in.
Mine has a dip stick in the cap. I also marked the outside of the reservoir with HIGH and LOW marks by measuring the marks on the dip stick. For mine, FULL is about 1" from the top of the reservoir with the jacks up.
Mine did that exactly once as well and I know it's because the dealer overfilled the reservoir because the fluid level was right where it should be after it made a mess of the compartment.
Any chance the cap was just loose?