IMO the odds are pretty good that the problem is either a lack of hydraulic fluid or a bad solenoid.
checking the level of hydraulic fluid in the tank is simple. retract all rooms and jacks before checking. use a clean cloth or paper towel to wipe the dipstick after removal and then dip it again. since the other jacks are working and the reluctant jack is in front, I'm betting that you do have enough hydraulic fluid in the tank but it's an EZ first step.
the owner's manual should have a diagram that shows which solenoid is which. if not then you might want to give Winnebago Tech Support a call. They're very good at helping owners diagnose problems. 1-800-537-1885. You'll need either the VIN of the chassis or the serial number of the coach. both should be on a placard inside the coach or on your owner's manual.
Once you've identified the solenoid make sure that it is closed tightly. Since you have the part have the tech replace it and then test the jacks. if it were me, I'd watch him make the swap. If the jack still doesn't work have him explain, in detail and in plain English, what he means about a "valve". another possibility is a bad jack. the jack might be damaged, or dirt might be preventing it from fully extending. those are possibilities but, in my experience,34-seasons, it is most likely the solenoid.
good luck and safe travels.
Rich
Ham Radio, Sport Pilot, Retired 9-1-1 Call Center Administrator
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2016 Itasca Suncruiser 38Q
'46 Willys CJ2A
'23 Jeep Wrangler JL
'10 Jeep Liberty KK
& MaggieThe Wonder Beagle