Welcome to the world of Jayco, what you are finding is very,very common and has been the source of many threads on the subject, a search will bring up many with great detail but the quick version is if you look at the mains inlet line you will find a large fitting of brass and plastic, this is the PRV/PLV, pressure reduction/limiting valve which is supposed to limit your water pressure, this often causes problems with pressure their are 3 ways to fix it......
1...........Remove and replace with another, available from Bunnings as they are just normal household PRV/PLV's but check the size of your line connectors to make sure they match..........check the flow thru of the unit before you bin it though, reverse flush as it does have a filter inside.....well most do....
2...........Remove, dismantle and stretch the spring inside while this works it is not always reliable and the spring may reform.
3...........Remove dismantle, throw away the loose bits and put back together and refit..........you still get a pressure restriction but the flow will be normal, I have used this method on 2 vans for a number of years without any problems, haven't found any places that have any pressure to worry about, the Guest fittngs and lines on a van are good for 150psi if fitted correctly and the HWS has it's own valve......
Always run the mains tap then flush your hose before you connect up to van, thusly ensuring you won't be pumping a bugs nest into your van when you turn the tap on and clogging everything up, coil your hose up and connect the ends together to keep out bugs and dirt when not in use.
It is a good idea to get some spare John Guest fittings, a spare NRV, joiner and elbow can come in handy along with a small length of pipe, colour doesn't matter as they are all the same specs............
