I hope i understand you correct, shoot me if i am wrong
You have a mobile home/van with a dish mounted. You dish only receives signals from the satellite, it doesn't send out anything, so there is no signal FROM your mobile.
What i can imagine is that the dishsize used on a mobile-home might be smaller then the ones used in normal situation (just guessing, don't have any experience with mobile-homes), also since a mobile home is not as stable as a solid house, the dish might move a little, the dish might be alligned less acurate, or even the power suply might be less compared to a fixed powerline. All these things CAN influence the quality of the reception.
But back to your question why some channels work and some don't in the evening: not all transponders have the same signal strength, so it can be that one channel can't be received while others can on the same satellite.
If this transponder is just in the grey zone (signal just not good enough) it sometimes helps if you get a better LNB, you could try with an low noise LNB, like an Invacom 0.3dB. This sometimes is just the little extra you need.