Only way to fill water heater with water is to fill rv plumbing system with water.
Course you could.........close cold water inlet (and hot outlet if you have one) then remove PRV and with a funnel pour water into WH tank. Then re-install PRV after you have filled it and fire it up.
You can fire up gas side without water in tank......valve in propane, light off stove top burners to get propane flowing and then turn on WH on 'gas'. Listen for it to 'click' (spark) and then listen form main flame. After you hear main flame light off and stay lit then turn it off. Won't be enough heat generated to do any harm.
Then later on when you have water in system you can test electric heat side of WH. At least you'll know that gas side works.
Is it time for your medication or mine?
2007 DODGE 3500 QC SRW 5.9L CTD In-Bed 'quiet gen'
2007 HitchHiker II 32.5 UKTG 2000W Xantex Inverter
US NAVY------USS Decatur DDG31