Regulator Mounting

short answer is yes and no :(

this is a simplistic description as to what is going on to help you see what the pro's and con's are.
the regulator adjusts the input volts to suitable levels to charge ( and prevent overcharge ) of the battery. ie solar panel output volts is typically more than you want for the battery.
the regulator can only see it's output voltage at the regulator terminal ( for most simple regulators, there are remote voltage sensing regulators but i'll ignore them for the moment ).
Now if you have any current flowing through a piece of wire there will be a voltage drop ( all practical pieces of wire have resistance ).

so:
solar panel ( Vin ) ----- wire ( Vloss-in ) ----- regulator ( Vcharge ) ----- wire ( Vloss-out ) --- battery ( Vbattery ).

depending upon where you put the long piece of wire wiill determine where we lose some voltage. as we want the regulator to look after the battery it'd be better if we kept Vloss-out to a minimum.

also electronics don't like lots of heat. the back of a solar panel sitting in the sun can be a tad warm. :)

so it'd be better to have the regulator close to the battery.

on the other hand it can be convenient to have the regulator on the back of the solar panel as it's one less seperate item to cart around or find somewhere to mount.

i'd probably put it closer to the battery than the solar panel.

To answer your question - YES - you can mount the regulator to the back of the panel with white silicon adhesive / sealant - That is OK to do -

However the response below is correct that it is better to place the reg as close to the battery as possible to avoid voltage loss over the 5 or 10 metre cable ..

Cheers

Mandrake

there is another couple of points to consider when deciding to mount voltage regulators on the back of solar panels.
i am speculating on this a little but, i would expect that temperatures of 50degC plus to be fairly easily achieved in full sunshine. perhaps more isn't unreasonable.
a lot of the cheaper regulators use commercial grade components that are only rated to ~ 60degC. most regulators will generate some heat in normal operation so you could be operating outside the devices temperature limits.

the other issue with temperature is that ( once again ) a lot of regulators are specified at 25dec. they have built into them temperature compensation ( batteries should be charged at differing voltages depending upon the temperature. higher temperatures require lower voltage ). i hope you can see where this is leading :)
now, if the regulator is parked in the direct sun and the battery attached to it is in the shade in a battery box under your camper, it's a pretty fair bet to say that they will be at differing temperatures.

So with the regulator mounted on the back of the solar panel you now have a voltage drop between the regulator output and the battery due to cable losses and a regulator reducing it's output voltage due to an increase of temperature.

this are all factors that should be taken into consideration when you build a good solar system.
the impact they may or may not have depend on various factors such as current, wire resistance etc.
which comes back to my original answer of yes and no.
more information is required to calculate if these factors are significant or not in your situation. i couldn't say that 'yes' is an ok answer to your question with the information given.
i would suggest that given that you have the option with an unmounted regulator that you do not mount it on the back of the solar panels, mount it near your batteries if practical.
cheers

My panel came with the regulator mounted to the back and from advice from the forum I moved it to the other end of the 10m cable...this has improved voltage drop from the battery to the controller

Thanks for the responses people. As my panel only came with a 2.5 mtr cable I am not sure if voltage loss would be an issue, but I do see where you are coming from. The different heat between the regulator and battery I understand so am thinking I may store it in the panel and make sure it is in the shade when in use.

Daryl