Hello and Welcome..
My name is***** happy to help / advise.. Sorry to see you have had a wait...
--Yes you do need to be checking on the rev counter if you are changing down the gears.. It will not like going into 2nd gear if the revs are high... it depends on the gear ratios somewhat as to what rpm to look for - but anything above 2k and it will struggle.
If its not going down hill - if its going up hill you mean you are having the problems with - then I assume you are saying, it doesn't like going into 2nd gear... as when you change it in, the vehilce slows down too much and you have to go back into 1st...
Its difficult not being there to check on this - but --it is a horsebox so its heavy... and it all depends on the gradient of the hill.... but if its a steep hill.. then you are going to have to get the revs way up...around 2k before you change into 2nd- and possibly even 3k... If you cant do that -for whatever reason - the gradient of the hill / the weight of the truck etc - then it would be normal to just drive in 1st up hill until such a time as it will take it.
Thats how I see it from what you have outlined - a tip really would be to have a local mechanic from a garage just take a quick test drive with you ... he will with his experience be able to feel / hear if there is a specific problem with the vehicle that needs attention... or if it is just as I have said above.
Let me know how you go..and / or let me know if you need anything further on this..
~~~~ Meantime - Please do click on the "Exc / Ok rating" for me at this time.. "Thank you" in advance for that .
.. Note that you can get back to me anytime at all after doing the rating - it doesn't close you out.
-- Best Rgds, MIKE.