Shurflo seems to be the pump of choice followed by Flojet. I replaced my old belt pump with a modern Shurflo. Look at the GPM (gallons perm minute) and also what pressure the pump is designed for.
Most RV's run in the 30-45 Lb pressure range. As a guess I would say 45 is pretty common. As long as you are looking into a pump, consider an accumulator tank as well. Those tanks can adsorb some water hammer (excess pressure when the pump kicks on) and some expansion pressure from the water heater if installed right. 6 gpm may be a bit high and 1.3 a bit low. Depends on the amount and duration of use. For my 25 ft. I use something that is only about 3 gpm. If you want more water from the shower some more gpm's would help (assuming the pressure is not maxed out).
Make sure any pump you get has a built in check valve. If it doesn't, then get one and install it on pressure side of the pump. Take the time not only to look over the pump replacement but check over the rest of the plumbing including the drainage system.
I am starting to replace an entire system because one of my problems was the pump not doing the job and in the course of repalcing it I got a chance to look over everything and found some interesting cobble jobs
. Rather than repair questionable practices, it isn't that bad (except for the new heater) to just repace the system with a better designed pex system. Hopefully all you need is a pump.
Check out prices all over before buying. If you want to save a $, sometimes your RV dealer will have some used parts form mods or wrecks. (The ones that never repack whell bearings and so forth---they are an excellent source of parts for the rest of us!
). Back to the subject---do a little research first. There is a lot of slack in the operational ranges that the AS's operate in, know what suits your needs top end is not necessarily a requirement. That said at least one excellent pump has already been recomended, and the $ for that model give a lot of pump for the $.
Oh and don't hook the pump up directly to a copper line--use flexible plastic.