I'd go with the RV style. That way if your RV pump goes out, you have a spare on hand.
If you are using a bladder in the back of the truck and the bladder has a spigot in the bottom of it, I doubt a pump is even necessary. It’ll likely flow out faster by gravity than pumping it.
I've never had an RV where the fresh water tank fill sits below the level of the pickup bed (maybe you have a jacked up mudbog truck?). Also, it won't work for washing the trailer.
Not sure I understand why it would not work for washing the trailer. I have seen some of the pumps that put out 70 psi. That is as much as you have in your house usually.
Also,I am not certain that the truck bed sits higher than the gravity fill on the trailer. Not saying that it does not but I am not sure.
With a pump certainly but it was suggested that a pump wasn't needed. Gravity draining the bladder out of a 3ft high truck bed isn't going to spray 10ft up the side of a trailer.
Similarly most fresh water inlets are around 4 feet off the ground, so gravity fed isn't going to work unless the bed is jacked up pretty high.
So you’re an intelligent man (albeit snarky)
So I have a 3’ x 3’ x 3’ bladder full of water, sitting in my 3 foot high truck bed, connected with a short hose to my 4-foot high fill port. Now, are you saying the water won’t flow into my Rv water tank via gravity/siphon? And therefore should buy a $70 pump?
Liquid seeks it's own level. So your 3 x 3 x 3 bladder in a 3 foot bed puts the top of the water level at 6 feet. So yes water will flow out of the bladder and into the 4 toot high fill port. At least until two vertical feet of water drains out of the bladder making the surface 4 feet off the ground.
From that point you would need either a siphon effect to remove the last foot of water out of the bladder. Or you would need to find a way to put pressure on the bladder to force the last water out.
But the flow rate would be slow and not under pressure. Which doesn't address the original specification of using the water to wash the trailer.
