I would think if you utilize the outside shower for the source, you would need the FW pump running. Should still work.
But if it is going into storage, then you need to drain the FW tank, so why not put a hose adapter onto the drain line and the run the tank dry from that? The pressure washer does not need a pressurized source.
So the short answer is yes, you could use that water.
Ha! You are correct that low flow would cause problems (cavitation) in the pump. :-/
Now I was thinking the OP would only have a small FW tank, so you would use a smaller pressure washer, but I looked up the OP and see he has a Miramar and that has a 100 gal tank! Utilizing the in house pump may be the answer.
This is what some on the Forum are using
$79 at Home Depot
Either connect to the outside shower with extension hose or directly out of a 5gal bucket.
Hopefully that $80 includes an extra battery or 2, I don't think 1 would do the entire motorhome.
Ryobi 18v tools are great, I have several different ones & I think my son has ALL of them with a wall full of chargers & batteries.
Danny & Linda
Sold our '13 Thor Redwood 36 FB
Currently RV & truckless