If the valve has a removable 'bonnet' (that nut under the handle) remove it, wind out the stem and gate and clean out any crud and try it again. Also, there is sealing packing of some sort on the valve shaft that may be too tight or broken and something you should be able to see and then either fix or replace the entire valve if necessary. Minimal tools but now days, probably metric sizes for wrenches. Then, of course, are those cheap, usually plastic, valves RV builders seem to love that don't disassemble ....
Dave W along with my DW, Susan and our poodlepups, Callie & Molly,2011 Ford F250 6.7 CCLB, 5er Hitch Option w/B&W Hitch,,Ride Rite air bags, 2014 Montana High Country 343RL (38')