When that happens on my Aliner it's usually air in the system. I generally try to prime the system by using a city water connected hose (including the pressure reducer). It takes a few minutes for the hot water tank to fill. I turn on the heater so I know it's actually hot water moving through the system. I leave the hot water faucet open at my highest system point (kitchen sink). You'll hear the air coming out and then maybe a little water. There's still air in the hot line. I'll close that faucet and move to the next step.
In my case, it's my outdoor shower/rinse unit, which is lowest faucet. Cold and hot - again generally - one at a time. Once I have decent pressure there, I'll close them, and move to the interior shower (mounted just above and facing inboard/adjacent to my toilet). I let that water drain into the gray tank. Once that's flowing better, I move on to the kitchen sink. Again, leaving the shower faucets closed.
Regrettably, it might take quite a bit of water to burp the system until it's flowing properly. That's the main reason I try to fill the lines at home before heading out. I'll drain the water and heater tanks to save weight before travel. I'm still uncertain if road vibrations help with burping the system. I've also wondered whether tipping the nose up might dislodge more air - TBD.
In addition to burping the water system as recommended by @David Blackwell… Make sure both faucets aren’t open at the same time. Close the shower faucet and open the sink. Check to see if any water comes out. Then do the shower faucet while sink faucet is closed. You can’t run both faucets at the same time.
For boondocking or camping in campgrounds without water hookups. I found a good gadget to use when filling the freshwater tank and hot water heater. I wish I had this with DiamondGirl. I used it at the gas station while filling up KODI. It lets you know how much is being filled into both areas. “Save a Drop” water meter. I’ll order one for KODI’s water accessories tub. Usually we fill up until water overflow back out of inlet. But knowing how much gallons had been filled is helpful.