Could be trash in the filter. There should be a simple screen filter between the tank and pump. If you had "junk" like plastic shavings, etc. in the pump from manufacturing, then it may have accumulated over time in the filter, making the pump work extra hard to pull water from the tank. It's about 4 inch round plastic thing with a clear cover. You unscrew the cover and there is a screen wire filter in there to clean out.
OR, if you've run the pump dry for too long at some point, I've seen the rubber impellers in pumps melt slightly - and they no longer pump well. They are replaceable but you have to pull the pump out, disassemble it and order parts.
Buddy Ray - Atlanta
Jayco Eagle HT 26.5RLS
Ford F150 Lariat, 3.5L V6 Ecoboost
Max Tow Pkg, 36gal tank
Reese Sidewinder and Reese Titan 16k hitch